Losing it slowly but all at once

So my husband and I split in July. I started seeing someone beginning of October. We slept together twice in the week we where together. Well this week he not only went back to his baby momma but I'm now 12 days late. The first test I took the positive was so faint. I'm in shock. My husband and I tried for 3 years with no success but this new person comes into my life and I'm looking at a positive test. I haven't told him, his baby momma keeps harassing me over him. Should I hold off on telling him? How should I go about it? How do I handle her? I'm high risk and she is stressing me out so much.