Today my Two year old bit my 3 month old on the foot. What do I do and how worried should I be?

Do nothing. They are still young. Jealousy is normal. Keep on taking care of both.. and do not exaggerate on punishment if ever you consider so!

At two, biting is probably more about instinct or rash decision than deliberate result of jealousy. If the 2 year old can communicate, try to find out what might have led to the incident. Baby invaded space maybe? Took something?
Try to communicate with the 2 year old on a level they can understand about being very careful not to hurt others (baby) and to talk with mom about something if feeling upset with baby. Something tells me the 2 year old might get the bite returned from the little one in a few years.