Had a concussion a year or so before. last night someone whipped a ping pong ball and it hit the back of my head? should i be worried?

Probably not: It is very unlikely a ping pong ball would create enough force to cause a concussion, even if it was hit very hard. Observe yourself for headache, dizziness, nausea, vomiting, memory and concentration problems and other post-concussive symptoms. Seek medical attention if these develop and last beyond a day. If I can be of help then join my care team at www.healthtap.com/dr-clarkeholmes