Bulge at bottom casing in area of extension ports

Hello All,

Basically you can see what is the problem on pics attached. This bulging of bottom case (the motherboard part) I have no idea when and how it is developed into this, just noticed recently. Moreover, now I have these linear bulges sticking out on the surface. It is Aluminum MacBook of late 2008 so it is not related to battery. I have removed the bottom case, have not found any possible troubles, all the screws are fixed well, yet I can not press the case to align it with the top case if the bottom case would not be flat enough. Also, I can fix the lower case at place with no problem at all.

Any ideas are welcome.

Thank you in advance,

Ivan