Is this just natural as the tomato forms or is it a sign of incomplete pollination?

It could be either and only a few days wait will tell. You may be seeing a young fruit or merely seeing the unpollinated ovary after the petals fall off. No way to know for sure without seeing it.

Given that your plants are in containers if I had to bet on it I'd bet on pollination issues due to watering and nutrient issues - both very common with container plants. It is a totally different forum of gardening than in ground is and has its own set of issues and solutions.

Container plants need regular feeding throughout the season as the nutrients wash out of the containers every time you water. Weekly feeding with a 1/2 strength dilution of a liquid fertilizer is commonly recommended.

Plus granular/powdered organics don't work in containers because the container mixes are sterile and lack the soil bacteria needed to convert the nutrients to a form usable by the plants. I assume you used a soil-less mix in the containers as recommended?

There are any number of liquid organic fertilizers available - kelp and seaweed extracts/blends, fish emulsion blends, worm teas, etc. available. This link will introduce you to just a few of them. And you can learn much more about any of those that are recommended over on the Container Gardening forum here. It all depends on what is available to you locally.

Hope this helps.

Dave

Here is a link that might be useful: Container Gardening forum