I am 20 now but my penis still as same when i was 13....what sex problem could i have?

Likely none: You may have no problems at all. If you had early puberty, your penis may have finished growing by 13 and is completely normal. If you haven't gone though puberty at 20 you need to see your provider. If you are asking if you have a small penis, have it evaluated by your provider. Even with a smaller phallus, however, you may have no sexual problems whatsoever.