Go to the specialist, you probably need a root canal.
Some background here:
Your mouth is disgusting (not just you, but everyone's). We fill it with food, we put all sorts of crap in it (and those of you who don't wash your hands after the bathroom, you literally put crap in your mouth when you touch something then stick it in your mouth, or chew on your nails, etc).
Point is, there is a TON of bacteria. Then you don't brush or floss and you get a cavity. The thing about bacteria, is it is really small. We have methods to get rid of the bacteria, but they are not fool proof. They work most of the time. Sometimes, before you even go to the dentist, the bacteria is already in the nerve. It may not hurt, but it doesn't mean it isn't infected. So, you go to the dentist, everything feels ok. The dentist tells you that you need a filling, you get it done. Unfortunately, the bacteria had already made it to the nerve, and you need a root canal.
That sucks.
Next, every filling you get done is a surgery. It's not cool surgery that you get flowers and dinner from your neighbors, but it is surgery. Sometimes you have surgery on your tooth and the tooth just gives up, the nerve dies, you have pain and you need a rct.
Those aren't the dentist's fault. It's called **** happens and unfortunately it happened to you.
Everyone could put dentist's out of business right now if you would eat right, drink water, and brush and floss. But we don't (me included), we get cavities and I still have a job.
NOW, (if you made it this far), on to your situation.
A couple of things could have happened:
1 - the decay was close to the nerve, the dentist tried to do you a solid and fill it when he should have done the root canal from day one. Maybe the dentist did an absolutely amazing job on your filling and it's a miracle it lasted this long before the root canal. I don't do people "solids" like this, because then when I do a great job and it doesn't work, somehow it's my fault. I'd have to see the X-ray to know if this was a possibility.
2 - the decay seemed small, the dentist cleaned the decay out, but unfortunately the bacteria was already deep enough into the tooth that a root canal needed to happen.
3 - your dentist is a hack, ****ed you up, and now is pawning you off on a specialist. That could happen, but it happens A LOT less than you think. Odds are, 99 times out of 100, it was one of the first two options.
The root canal isn't a bad procedure, it will probably be expensive, you will need a crown and buildup afterwards. Your other option could be pulling the tooth.