Hi, and thanks for your medical question on eHealth forum!
In all probability what you have is likely an abscess or a boil. This can happen due infection introduced by anal sex. Normally, this would need anti- inflammatory analgesics and antibiotics and at times the pus needs to be drained. Since you are with the same partner, chances of genital warts are less.
However a confirmed diagnosis cannot be given on the internet. So, if ER is your only option, then you have to go there. Hope this helps. Take care!