From Monistat's page -
Can I use a condom while treating my vaginal yeast infection?
Do not have vaginal intercourse while using MONISTAT®. When using MONISTAT® do not use tampons, douches, spermicides or other vaginal products. Condoms and diaphragms may be damaged and fail to prevent pregnancy or sexually transmitted diseases (STDs).
My guess is that there are oils and other products in the treatment that will cause damage to the condom, probably tearing.
Also, let your body heal. If you still have any symptoms of the yeast infection, you shouldn't have sex. Your partner could get it, and then you'd end up just passing it back and forth. Even when using a condom, this could happen, as your skin will come into contact with his skin.
Hang in there. Yeast infections are no fun, but it will be over soon.