I would recommend going to see your primary care doctor
about this issue. You are definitely right that a mono infection can cause the formation of pus in the back of the throat. However, since you've already had a mono infection, you should be immune and this shouldn't be the cause the pus in the back of your throat this time.
This means that other infections need to be ruled out. Most importantly, it is necessary to rule out a strep throat infection. Strep throat is caused by a bacteria that can infect the throat, and it is one of the most common forms of throat infection. It can cause pus in the back of the throat, together with a usually quite severe sore throat
. It is important to diagnosis the condition, because it needs to be treated with antibiotics.
Your primary care doctor can examine your throat and can take a swab to test it for strep throat. If this is positive, then they will be able to give you a prescription for antibiotics to help clear up the infection quickly.
Make an appointment to see your doctor at your earliest convenience. I hope you are feeling better soon!