That tomato is showing blossom end rot (BER). It's a failure in the plant to circulate calcium effectively to the developing fruit. It is most commonly caused not by calcium deficiency in the soil (unlikely in your case if you have been using tomato fertilizer regularly, as most tomato formulas contain plenty of calcium) but by watering problems, either too much, too little, or erratic watering. Cold and heat can also contribute to the problem. Some tomato varieties are very susceptible to it, and tomatoes in containers have more of a problem with it because it's harder to keep moisture levels consistent in a container, particularly if the container is on the small side, as a 7 gallon container is, and the roots have difficulty doing their job when they heat up too much, as can happen in containers in the summer heat.

Often, the problem is transient, i.e., it will show up on a few fruits for a short time then stop as the plant gets better at coping with fluctuating conditions.

Make sure you check soil moisture before watering--don't just water on a schedule regardless of what's going on in the soil. Check a couple inches down--don't go by what the surface of the soil looks like. If it's still nice and moist, don't water. If it's starting to get dry, then water slowly and deeply, i.e., deeply until the water runs out the bottom but not so quickly that the water runs out before the soil can absorb it. You may be watering too much or not enough--it will depend on your climate and conditions. In a climate that gets frequent rain and isn't very hot, daily may be way too much water. In a hot climate, a large container plant may need watering more than once a day. Or you may be watering too shallowly, which is a big contributor to BER.

Flowers getting smaller as the plant puts on more and more fruit is normal. Flowers drying up and falling off, i.e., "blossom drop", is quite normal when the plant is under stress, especially hot weather with temperatures over 90. It is also normal for a plant to abort flowers when it has a heavy fruit load on it, and even under the best conditions, no plant sets fruit for every single flower it produces. It's unlikely to be directly related to the BER--it's just that very hot temps can be a root cause for both BER and blossom drop.

Thanks for you answer, gorbelly. You think I should cut all bad fruits? There is no chance for them to recover, right?

Yes, you should discard affected fruit. They will probably get some kind of mold or ants, and they will taste bad. The calcium problem that leads to BER actually happens a long time before it even shows up. Once you see it, it's too late to do anything about it, unfortunately. But, also, in many cases, once you see it, the problem that caused it is already resolved or passed.