What are different cause of lumps inside your skin...

hmmm,

There are many causes of cysts within the skin. Epidermal inclusion cysts and sebaceous cysts are probably the most common. Localized bacterial infections can also cause lumps, but they are usually red and hurt. There are other infections that can cause lumps also, and can cause enlarged lymph nodes. There are some lumps which form on the superficial peripheral nerves. Ganglion cysts are very common around the wrist, but can be seen around any joint or tendon in the body.

Of course, there is always the very remote chance of a lump being a tumor. But, again, these are kinda rare.

If the lumps do not go away, or they develop other symptoms, you may want to have your physician take a look at them.

Good luck.