First time tomatoe grower. My two largest tomatoes are rotting on the bottom.?

Question

My plants look healthy are in pots. So what can be causing them to rot.

Answer 1

They are getting too much water causing bottom rot.

Answer 2

The problem you're seeing is called blossom end rot.  The ultimate cause of this affliction is a lack of calcium.  If the soil is still cold when the plants were put in the ground, it prevents the plant from taking up any calcium, even though calcium may be available in the soil.  

The good news is that this is typically a temporary problem, afflicting only the first set or two of tomatoes.  Just pick off and discard the afflicted fruit.  You'll find that the later fruit sets will ripen just fine.