Squiffie I had a tiny bit of blood on a tissue yesterday. I worked out that I'd be due a period about now if I wasn't pregnant, so it could have been a tiny trace of my body trying to follow normal pattern by mistake. Since you're at the same point as me, it could be similar for you?
Agree also with the others that it could easily be that you've just done a bit too much lifting and carrying, and caused a minor internal bleed. Alternatively, is there any chance it could have come from further back? Because of the increased blood flow, you could have irritated a bowel and get a bit of blood that way. Or, the ever delightful piles could be starting to appear.
Get a pad in when you have access to one and then make sure you rest with your legs up this evening. Hopefully there'll be no more or very little blood. If there is a bit more, or you're still concerned in the morning, then phone the EPU first thing.
The fact that you had a HB last week is good, I think a HB and expected measurements at 7 weeks is a 97% chance that all will continue fine. So there are pretty good odds that it's all going to be ok.