There's something forming on my ribcage.. like some sort of tissue?

You don't say how old you are , if you are in the older age bracket then more fatty tissue can build up around the body ,you must be slim if you can feel between your ribs . The best thing to do for your peace of mind is go to your gp and see what he/ she says or have a chest exray and put an end to your wondering , You may be only imagining it because if you are constantly poking and looking at it you are going to make yourself sore its probably nothing much ,good luck