It sounds like Blossom End Rot.
Blossom End Rot is a physiological disorder, cause by the plant not being able to metabolize the calcium in the soil. Usually, the plants got too wet or too dry and the plant could not metabolize the calcium. Unfortunately, the problem can occur weeks before you see the brown end. It is usually not a lack of calcium it is just they can’t absorb it.
- Try to keep the soil evenly moist not wet and never allow the plant to dry out.
- Avoid high nitrogen fertilizer liquid fertilize. It can block the calcium.
- Check the soil pH and make sure it is not low or high. It should be around 6.5.
- If you are in a cool area, make sure the soil is not cold when you transplant your plants.