I slept with a girl and she pulled my penis upward when fully erect and there was a vein on the side of the penis that hurt andnow my erection is weak and the sensation on the penis is weak too?

It is not feasible to provide a meaningful opinion without additional history, physical examination and may be some tests. It would be prudent to see a doctor. Wish you good health!

Your penis got overstretched and should be normal in a few more days. From your description no major harm seems to have occurred. I Think you will be OK. But if not better in a few more days -get checked by a Urologist