I’ve a big spot on my penis. Is something wrong?

On the penis are sebaceous glands that secrete oil: sebum. The opening of a gland can become blocked. The oil the gland keeps producing cannot escape. The sebum that is trapped hardens into a yellow cheesy lump. If you try to squeeze it it often becomes worse. So you better leave it alone. Normally it will disappear by itself. But if it becomes very big and hurts you can go to your physician for help. If you had unsafe sex a spot can also be something more serious, like syphilis. In that case you should seek advice from your GP. If it does not disappear after one or two weeks, you better go to your physician anyway to check out if it really is a pimple.