Abscesses don't tend to be sore if they're draining. The pain tends to happen when they stop draining. I take it you had xrays of the teeth/tooth next to the lump? Before all this happened, did you have an overly sensitive tooth?

The thing with toothache, and something a lot of people tend to not know, toothache never ever gets better or heals. When things stop nagging or the pain ceases, things have just gotten a lot worse. It means the tooth has died, and the pulp inside the tooth has began to decay and all that rotting tissue has nowhere to go. Because blood can't get into the tooth once it's died, your body can't rid it of infection, so you get an absecess.

Thank you for your reply! The dentist did take standard x-rays of each side of my mouth but it was before I mentioned the lump because as soon as I went in to the room they said the x-ray machine was being rebooted so they had to do x-rays first. I had a slightly sensitive tooth/roof of mouth but if it was just an abscess then wouldn't the antibiotics have cleared it up for now? They seem to have done nothing. The tooth that it's near I have had a root canal done in previously so I don't know if that makes any difference 

I have been back to the dentist today and she did more x-rays specifically of that tooth and said that nothing looks really clear. She has drilled out the filling to allow any infection to drain if that is the cause and has booked another appointment for Monday. 

If the lump has gone by Monday then she will remove the tooth and that's the end of it. If it hasn't gone then she is going to make a referral to the hospital under the cancer pathway

I worked in a dental setting for years (wasn't a dentist). The thing about xrays, they're only good for giving a 2d image of the tooth. Awful giving a true picture unless there's noticeable decay or bone loss under the root. Bone loss under the tooth is the infection eating away the bone, thus it shows up as a small dark pocket under the tooth. Xrays will not show an infection on the roof of the mouth. The infection needs to be under the tooth. Infections find the path of least resistance to the surface, and where that path happens to be on any given tooth infection is random in each person. But they do tend to follow the same path depending where they are.

As for antibiotics clearing tooth infections, no they don't. Antibiotics require a blood supply to reach their destination. Because no blood goes into a dead tooth, it's impossible for antibiotics to get to the root cause of the infection. The only way a tooth infection is eradicated is either via a root canal (removing the dead tissue from the tooth canals) or extractions.

A cone ct scan is by far the best scan to get a true picture of any given tooth. It gives a 3d image and can detect infections that xrays can't. Xrays are as basic as they get, but they're cheap and easy to use. Head cts used for teeth are expensive and not many dentists have one. You usually need to go to a dental hospital for that unless you paid prive.