Welcome to MedHelp's Pain Management Forum. I am glad that you found us and took the time to share your medical concerns.
When we "find" a lump it can be very frightening. We obviously think the worse...but more often then not they are not life threatening. However it does require medical evaluation. So I encourage you to consult your PCP.
You may also want to post on our Undiagnosed Symptoms Forum. Here is the link:
Please know that you are always welcome here.
I cannot make intelligent guesses as to what your "lump" may be. There are many possibilities. Obviously one would be, as you indicated, a gas bubble. It could be a muscle spasm, slow moving stool....or again so many things. So please make an appointment with your PCP and check into it.
I hope you will let us know what you discover. I'll look forward to hearing from you.
Best of Luck,