I can see cracks over foreskin after each erection.

Infection? Eczema?: Several kinds of dermatitis could do this: bacterial or fungal infections, eczema, or other inflammatory conditions. See your doctor -- or if s/he isn't up to speed on skin conditions, ask for referral to a dermatologist. It should be a quick and easy diagnosis and simple treatment. (In case you're wondering, the large majority of genital skin rashes are not STDs.)

See PCP: Although it is not likely a STD you should see your PCP and have the rash looked at.

Phimosis: If I understand correctly you have your issue when you have erection and your prepuce is retracted behind the gland. If you are diabetic this situation could be more often, if not I would also suggest check your glucose levels to discard that. You could be suffering from phimosis, there are steroid cream that could help but often it is recommended circumcision. An urologist need to check you.