Its nearly impossible to tell this early, espcially if she is still in heat. Even if she's not pregnant, she can still display signs OF pregnancy, such as lactating nipples, etc. Jamie went through something similar and gave me a scare, but she ended up NOT being pregnant.

If she is still in heat, she CAN NOT be spayed until its over.

The only way to tell for 100% certain, aside from time and waiting it out, is to bring her to the vet!

For reference though when Jamie went through her heat, her nipples got large and hard and gained some weight, too. But its a part of the normal cycle I assume and a week later she went back to normal.