Toddler neck lymph nodes

My 2 year old had two enlarged lymph nodes on his neck. Just noticed them. I’m wondering if I should be worried?

He got a stomach bug last week Monday. He threw up once at the beginning, and had diarrhea all week. Only in the last two days have his poops slowed down, and today he finally had one that was soft but not runny. So we are on the tail end of that virus.

And, on Monday, he was also bitten by a mystery bug on the back of his neck. It was small and black and didn’t look like anything specific, no legs or anything. I brushed it off when I saw it and it left a little bloody spot! He has been itching it and it is still red. Maybe that caused the lymph nodes?

Wondering when I should think about calling his doc