blossom end rot? caused by calcium deficiency? my guess...

Without seeing a photo and going only from your description I would also guess it is BER (Blossom end rot). It is fairly common in the spring given the weather.

Are you familiar with BER? If not there is a FAQ here I linked below and lots of discussions here about it. You can Google 'tomato blossom end rot' and see photos of it to compare to yours.

The key to controlling it is to stabilize your soil moisture level as much as possible. Is your plant in the ground or in a container?

Dave

Here is a link that might be useful: What is BER?

Richard

Check out some pictures of blossom end rot as well as reading about it to be sure, of course.

When you wrote "black hole in the bottom of my tomato", I wondered if you meant the black leathery patch that is BER or if you actually meant a hole, which is something altogether different, probably caused by an insect. The tomato tries to heal itself by growing tissue around the insect injury, and that causes dry rigid tissue.

In either case, the damage is probably only a one-time deal with insect injury or temporary damage if it's BER. Many people cut the BER black part away and eat the unaffected parts. Or you can just toss the affected fruit and wait for the weather to become stable. Then the BER will quite likely stop.

It could possibly be a "hole" because the BER got bad enough to cave in...just a thought. Control watering, keep it more even, not too much or too little, BER is caused by calcium not being transported to the end of the fruit, which is caused by irregular watering, so it's not really a calcium deficiency as much as an inability for the calcium to get through the whole fruit.