Hi Bethy ~ a number of things could be going on here. If I were in your shoes, I would take myself off to a medical laboratory and get a blood test done for pregnancy. Then you can relax one way or the other.
It could be that you are on hormone treatment to assist conception - in which case, skipping periods without being pregnant is not uncommon - AND most irritating when you keep on hoping and going for tests... The hormones given to aid conception can also cause pregnancy-like symptoms.
On the other hand it could be because you are wanting a baby so badly that you are experiencing a 'phantom pregnancy'.
Go for the blood test and if still negative, I would definitely see a gynaecologist for a good check up in case you have a hormone imbalance.
Kind regards
Jewel