Hi freddieG, and welcome! I've been having the same problem, and there's been a lot of discussion on the Forum about it recently. Sounds like your tomatoes are suffering from BER (blossom end rot). Put that in as a Search term at the top of the page, and you should be directed to lots of info.
What I've learned from reading comments on the Forum is that the problem can be quickly remedied by adding calcium. I think it's 1/4 cup of hydrated lime per gallon of water, mixed and then poured into the watering tube. I think it's standard practice to remove the damaged fruit. I did, since the affected tomatoes were still very small, and I want the plant's resources to be directed to the healthy fruits. Since I added the calcium, no more tomatoes on my plants show any sign of BER. I don't want to steer you wrong, so check what others have advised. Apparently it is important not to add too much calcium.