Getting that mare and stallion check for any infections that can prevent the mare from carrying a foal is very very necessary.
For the life of me I cannot understand why a stallion owner would not require you to bring a clean bill of health before breeding.
If I were you I would Ultrasound that mare. Not only for twins but to see if the breeding took and if so then the problem lies with the mare. The vet can then determine diagnosis the problem.
You have been given very good advice and with your limited knowledge and funds IMO it would be wise to heed the suggestions people are giving you.
If you truly want a healthy foal you need to take all the precautions.
Brit I have 32 horses. If I loose a mare or a foal it is not going to be very traumatic for me. Emotionally or financially My breeding program will recover. If you lost your mare and/or foal think about how you would feel.
Listen to the other members Britt. That is the intelligent thing to do. Shalom