Had root canal filling nearly 12 mth ago. Now have an abscess under tooth. Face swollen etc and now neck seems to be too. Taking amoxicillin on day?

See a dentist ASAP. Do not fool around with this yourself. An abscess needs draining and you should be seen as soon as possible by a dentist or oral surgeon and have this treated professionally. Good luck.
Compare x-rays. I would want the dentist/endodontist to compare x-rays then and now to provide a more definitive answer. Sometimes the lesion can be healing granulating tissue. A lot depends on if you have symptoms too- swelling, bump on tissue, pain upon eating, temp sensitivity.
Infection. You need to see your dentist. The abscess at roots end has not healed and you need to have it addressed.