As far as relaxants you've got herbs like Kava Kava, St. John's Wort, and Valerian Root that claim to relax you. Products like 5-HTP and melatonin are also good for stimulating calming seratonin responses. For you, I'd probably try Valerian root out - I've heard it works for similar symptoms.
Though, as far as what's causing the problem I'd consult a doctor. Sounds possibly like a chemical imbalance or hormonal issues.