A week before my period was due I was having worse cramps than normal but didn't come on. I did a test which was positive and said 3-4 weeks pregnant. I then started bleeding exactly a week after I was due on for 3 days although not as heavy as usual. The day I started bleeding I did a test which was negative. I think I have propably miscarried. Would you agree

It's possible... but what test did you take that stated positive and gave you 3-4 weeks? Were the tests you took both home tests or at the doctor's? I would give it a bit more time and retest, being sure to follow the directions exactly.