Taking the Plan B drug is up to you.

Yes, you can get pregnant just from the activity you describe. Not from sperm traveling through the skin of your stomach though, that can't happen unless some of it falls on your vagina.

A penis often has pre-ejaculate on the tip, long before ejaculation. This fluid can contain sperm, his having contact with your vagina, and being inside could deposit sperm within. It only takes one to find an egg to fertilize.

The chances are less, of course, but there is a chance.

If you are going to continue doing this activity, you should obtain a reliable birth control method, like birth control pills, or use a condom. Plan B is not meant to be used regularly, only as an emergency contraceptive.