http://www.medhelp.org/posts/Mens-Health/Penis-retracting-into-body/show/183912
Visit the above link in MH. There are more than 200 comments. First post suggests cardiovascular exercise and the second post support the cadio and suggest you to drink 8 glasses of water in a day. These are interesting posts. you will find solution to your problem.
Do you feel stressed or have anxiety?
How old are you?
It is normal for the penis to shrink to a fairly small size when not being sexually aroused.
If you get sexually aroused and the penis enlarges and is erect and you have no problem with urinating (doing a pee), then things are normal.
So stop pulling.