I interpret your "3rd cluster" to mean that you are just starting to have a new case.

If that's correct, it's not unusual for all elements of Cluster to change, often for many months, before settling down into a pattern which is stable, for you.

You didn't mention any of the standard meds used to abort a cycle and to use for long term prevention of them. And, what kind of a doc is treating you?
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Cluster headache.
From: Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to or (Orphanet Journal of Rare Diseases)
[Easy to read; one of the better overview articles I've seen. Suggest printing the full length article--link, line above--if you are serious about keeping a good medical library on the subject.]

Leroux E, Ducros A.

ABSTRACT: Cluster headache (CH) is a primary headache disease characterized by recurrent short-lasting attacks (15 to 180 minutes) of excruciating unilateral periorbital pain accompanied by ipsilateral autonomic signs (lacrimation, nasal congestion, ptosis, miosis, lid edema, redness of the eye). It affects young adults, predominantly males. Prevalence is estimated at 0.5-1.0/1,000. CH has a circannual and circadian periodicity, attacks being clustered (hence the name) in bouts that can occur during specific months of the year. ALCOHOL IS THE ONLY DIETARY TRIGGER OF CH, STRONG ODORS (MAINLY SOLVENTS AND CIGARETTE SMOKE) AND NAPPING MAY ALSO TRIGGER CH ATTACKS. During bouts, attacks may happen at precise hours, especially during the night. During the attacks, patients tend to be restless. CH may be episodic or chronic, depending on the presence of remission periods. CH IS ASSOCIATED WITH TRIGEMINOVASCULAR ACTIVATION AND NEUROENDOCRINE AND VEGETATIVE DISTURBANCES, HOWEVER, THE PRECISE CAUSATIVE MECHANISMS REMAIN UNKNOWN. Involvement of the hypothalamus (a structure regulating endocrine function and sleep-wake rhythms) has been confirmed, explaining, at least in part, the cyclic aspects of CH. The disease is familial in about 10% of cases. Genetic factors play a role in CH susceptibility, and a causative role has been suggested for the hypocretin receptor gene. Diagnosis is clinical. Differential diagnoses include other primary headache diseases such as migraine, paroxysmal hemicrania and SUNCT syndrome. At present, there is no curative treatment. There are efficient treatments to shorten the painful attacks (acute treatments) and to reduce the number of daily attacks (prophylactic treatments). Acute treatment is based on subcutaneous administration of sumatriptan and high-flow oxygen. Verapamil, lithium, methysergide, prednisone, greater occipital nerve blocks and topiramate may be used for prophylaxis. In refractory cases, deep-brain stimulation of the hypothalamus and greater occipital nerve stimulators have been tried in experimental settings.THE DISEASE COURSE OVER A LIFETIME IS UNPREDICTABLE. Some patients have only one period of attacks, while in others the disease evolves from episodic to chronic form.

PMID: 18651939 [PubMed]
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And the PDF file, below.

Read up on the Oxygen Info to the left. Pay close attention to the discussion on welder's O2. This is not a pain, it is relatively inexpensive (much less expensive than Imitrex). Welder's O2 comes over the counter no questions asked. Getting it from a doctor should be easy as well, especially from a doctor who knows what he is doing where CH is concerned. When I finally got to a neuro who knew what he was doing, he literally would not let me leave his office till I agreed to try it.
As for how it works when used correctly... It was so life changing, that the thought of having to deal with CH without it drove me to buy my own equipment. I made sure I would never be without it no matter if I got insurance hassles, moved, new job, new doctor to break in... etc.
As for the Imitrex injections, you might try dialing the dose back. Not sure what they gave you, but many find they can do with less than the usual 6mg dose, and can split it up to get better coverage and cost savings. See the Imitrex Tip link to the left.