I would think the suplement the vet was administering has worn off and all traces of it have left your horses body so her hormones are 'rageing' again. Thinking this through logically, she would have had 2 seasons in the 2 months (mares cycle every 3 weeks) but her first one after the suplement was taken away wouldn't have been so dramatic because there would still have been traces of whatever chemical there was in her body. Now there is no supressant to your mares hormones, she may well just go back to her old hormonal marish ways :/
Its not strictly speaking natural for mares to have to cycle through an entire breeding season. In the wild they would most likely be covered on their 1st or 2nd cycle and be pregnant for the rest of the year- steady hormone levels. Most mares are fine but some don't cope very well constantly cycling and constantly having hormone changes, hence why they're stroppy and 'mareish'
Do you know if it was a hormone suplement or herbal one? It sounds like it was a hormone being given if it was 'birth control'? I guess like the pill for humans. You could try a herb suplement to try to control any dramatic hormone swings. I know there are some suplements that specifically target moody mares that are herbs, not hormones, like NAF Oestress NAF.co.uk - Natural Animal Feeds
(Don't know any American brands, soz)
Why did the vet take her off the suplement? Perhaps its worth talking to him about having a marble put in her uterus so the body thinks its pregnant and stops cycling all together?