I just ran 2.5 miles in 40 minutes (which is a lot for me ) And i feel like fainting . could i have over done it?

Hmm...maybe. Have you been running much or did you just start? Maybe you are just having an extra tired day.

I mean ive been running and cycling but i haven't been eating much and i also have asthma .

Why havn't you been eating much?

You just answered your own question, darling.

Try increasing your calories a bit more maybe? Maybe you just didn't have enough calorie intake to handle that much output?