Hi
I would not worry so much about each and every move a horse has. You must have some trust and faith in God. I have never had a mare to abort that gave me clues/signs that was what she was trying to do. Now, if she is getting a bag when bred in May I would worry a bit about the breeding dates. Early foaling mares do not often give us a bag either.
She could be due as early as the end of April. I count the first breeding date and add 11 months. Have been correct about 99.0% of the time, within 1 or 2 days. This makes your mare due in about 330 days, which is suggested on the bottom of the foaling calculator on LB site as an exoected time to use.
There is a preventative if you think she is going to foal too early - and that is regumate. You must check with your vet first and see if this is recommended. Some say regumate works, others say it does not work towards the end of their pregnancy. I am living proof that it works at the end of their pregnancy.
Good luck.................in any case.