Wasn't this posted in bar..?
Depending on how thick the towel is, and how certain your ex is,
its possible. But there's no use worrying about it, wait for a few
more weeks and use a pregnancy test kit. Its cheap, discreet, and
readily available everywhere.
If you know you're going to be so worried, then why did you do it
back then? Just don't repeat this the next time...