Should we be worried?

My husband (29 y/o) has had a swollen lymph node in his left armpit for a couple of days. The first day he told me he was feeling a bit odd (like when you are going to get a fever, but he never did). Today he told me he was feeling fine but his lymph node was still aching. I examined it and you can feel a bump and the skin over it is red. He also just had diarrhea about 10 minutes ago and I’m starting to worry this might be something really serious (like lymphoma), as he doesn’t have any other flu-like symptoms. Should we be worried?