Variation in the length of menstrual cycles

Hi,
I'm 37 yrs. old and my cycles have been real irratic. Up until about 5 yrs. ago, my cycles were 27-28 days. All of a sudden, my periods stopped and I had to take provera/prometrium to get it started again. They were fine for about a year and then all of a sudden, no period again. Subsequently, I started fertility treatments for about a year, but no luck. During this time, they discovered a small grapefruit sized endometrial hematoma on my right ovary. I had it removed...it was attached in only one place on my ovary and had no other attachments to tubes, etc. The dr. said he examined everything else during the surgery and there was no more endometriosis and no other problems. My cycles were about 28-29 days for about 8 months after that.

This last year, my cycles have been like a roller coaster...23 days one month, 39 the next, 25 the next, 30 the next, and so on. Well this last month it was 37 days and then today I started on day 21 of this cycle. My periods are pretty normal as far as length and heaviness go. Is it normal to have all this variance in cycle length from month to month?