There are a few different possibilities here, but the most likely is that you simply have a condition called seborrheic dermatitis. Seborrheic dermatitis is an inflammatory condition of the scalp that is very common. "Dandruff" is actually a mild form of seborrheic dermatitis, but the condition can be much more pronounced, leading to thick greasy plaques of skin on the scalp, sometimes with small pimple like swellings. The condition is caused in part by dryness and irritation of oil-producing parts of the scalp, probably in most cases aggravated by colonization of these areas of the skin with certain fungi.
You may want to try washing your hair a bit more frequently and combing afterwards to moisten and loosen these dried and bumpy areas. Using a dandruff specific shampoo will also help, as it further will loosen up an thickened areas of skin and will also fight back any fungus living there.
If these methods don't work, you might want to go see your primary care doctor
. They will be able to take a look at your scalp and confirm the diagnosis (or suggest an alternative diagnosis), as well as prescribe topical medications that are stronger than what you can get over the counter.