You may just need to wait a little longer. Sometimes it takes time to even see a sac, but a thickened endometrium is a sign of pregnancy. Docs usually wait until about 8 weeks to do an ultrasound b/c you can see something for sure at this time. I usually start getting ultrasounds pretty early b/c I'm high risk, so on my first appt....just about a week after late period, all they saw was a thickened endometrium. The following week they saw a sac, and it just progressed. They could do a beta HCG test, where they take your levels one day and then again 48 hours later to see if they're doubling. If they do, then you're on a good path. Good Luck!
Okay now it maybe stress are sometimes your period can come late. but you will have to know if your boyfriend precumed in you.