I don't feel pregnant anymore?

I'm 14 weeks pregnant, for the last few weeks my stomach has been very hard and very big, I often get asked if I'm carrying twins. This is my second pregnancy. I was feeling slight movements for the last two weeks. However this week my stomach has gone really flat is no longer sticking out, I've lost the metallic taste in my mouth that I used to have and I haven't felt any baby movement. I've had constant backache but no bleeding. My next hospital appointment isn't until September. I'm really scared that there's no longer a heartbeat or somethings wrong.