The below picture hopefully will show you, this is starting to occur on the toms before they turn red, upon cutting them open inside, theres no bug that we can see but certainly some 'infection' going on inside, please tell us what you think, thank you.
Certified GKH Gardening Expert
This is, actually, a pretty simple fix! This is a calcium deficiency, which causes blossom end rot. Calcium also likes to be paired with magnesium, since they rely on each other for uptake.
I would recommend using dolomitic lime in the soil before, during, and after, as this prevents many problems that will plague tomatoes, in ground, and especially in container.
This article will give you more ways of controlling this issue: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm