That doesn't sound like a tumor. The fact that it's very small and you can't even find it anymore is actually very reassuring.
Tumors don't just disappear within minutes, and they're noticeable even when you're not actively pressing on them.
Just keep an eye on the area. If it starts getting larger and begins affecting the appearance of your breast, then it would be worth paying attention to.
To me, this sounds much more like an inflamed hair follicle, an ingrown hair, a tiny cyst, or a small area of irritation from friction. It could even be a small pimple under the skin, yes, even within the tissue in that area.
Also, I know you have OCD, so try not to keep checking or pressing on it. Repeatedly feeling the area can irritate the tissue and cause more tenderness or swelling, which can make it seem like there's more going on than there actually is.