Mare who ties up

[Wachipi@aol.com]

I've been a lurker here for about 2yrs, with occasional inputs, but now I've got a serious problem.
I have a 7 year old Qtr Horse mare (light muscle structure) who had her first mild tie-up at the end of June.  She did it in the trailer from Sacramento to Cool.  When we got to Cool, she couldn't back out of the trailer to get out.  We took her immediately to the vet where she got an injection of Banamine and a 10 day tapered course of Phenylbutazone.  The vet said it could be due to her bucking and running earlier that morning, and maybe cyclical with her estrus cycle.  She got a larger pen to exercise herself and was brought back by ponying and just the last three weeks was being ridden bare back for no more than 8 miles.  In late July I ran a full blood panel including Selenium.  All values were normal except for a low PCV of 29%.  Today she was ridden bareback at a walk for 3 miles of mostly level trail, some rolling hills.  She tied up again.  Caught it early, gave her Banamine IV and found a park ranger to get me back to my trailer to come back for my horses and my roommate.  All the horses are on Rye/Oat hay with paddocks to run in. Is it possible to tie up with heat cycles, is her diet ok.  All help is appreciated.
Shara Llewellyn