Is your cycle so regular, that you can be sure that the race will be just at the same time as your period?
I'm asking because last month I was fretting about the same issue just to find out that my body decided to be 2 days late - so there was no issue at all. If your cycle is just a little irregular, chances are that you won't have a problem at all.
If you have to deal with this problem: I talked to my doctor about this and she precribed a pain medication especially for pain/cramps related to menstruation (it's Ibuprofen-based). If I feel really bad, I take one in the morning and feel quite ok for the rest of the day, and I normally have really serious cramps that make me nauseous and dizzy.
You'd have to ask if it is safe to work out while taking medication though, I don't know about this for sure.
I would get medication and give it a try, but permit myself to not start/not finish the race, if I felt really bad. I don't think this is "feeling sorry for yourself", it's just sensible to not ignore serious pain.
Generally I feel that stress worsens my problems. I try to use this time of the month to be gentle to myself, take it a little easy and permit myself not to "feel like" doing something. This my sound strange but really helps for me.



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