Swollen lymph glands can be very common and are often a sign that the body is fighting an infection. They usually go down after a couple of weeks. The NHS have written some information on swollen lymph nodes which you can read about here
We are unable to diagnose (tell you what's wrong) so if you are concerned about the lump under your arm then do make an appointment with your GP.
If you say that you have put on some weight you may find it helpful to have a look at the NHS information on healthy eating which you can see here
Try not to worry but if the lump does not seem to be going away then do get it checked out by your GP.
