If you're not allergic to penicillin, that is the best antibiotic for your situation. As for when you get back, It's tough to tell without seeing you or an x-ray. It may need an apicoectomy or possibly an extraction. Apicoectomy is surgical procedure to clean out infection from the root tips end of the tooth. Good Luck.
I work in the dental field and see things like this periodically. If infection is present...ie a gum boil, swelling, fistula tract, ect...then an antibiotic will help. You may also try taking anti-inflammatory pain med..like advil, etc...or have a pain med prescribed. Most importantly please check with a dentist, doctor, etc... b/c you definitely need to address the tooth. Remember you can always get a second opinion on the rct treatment. Is the tooth crowned, broken, or does it still have a temp. filling in it? Good luck