My answer is the same. Get checked for STDs. Do not be embarrassed to ask for the test because this is your life we are talking about. No time to be shy.
We have lymph-nodes all over our bodies. Behind the neck is another place to find them besides armpits and groin.
If you are too shy to talk to you doctor about it which is a shame, then please go to anonymous testing.
May be you are just plain stressed out. Hopefully you have learned your lesson on casual sex like that.
All the best and please let us know what happens.
Always
Muthoni (Mson)