I havd red spots on my tongue and large ones on the back of it, what could this be?

Taste Buds. Provided that these bumps are not a symptom of underlying disease, i.e. There is no pain, swelling, bleeding, burning sensation, or discomfort of any kind, they are taste buds. The major test buds are in the posterior part of the tongue.
Taste buds\papaillae. I agree with the excellent answer given by dr. Gustavo ruiz de castilla. It's always best that you always check these things out with a dentist who can actually see what you are describing just to be sure. Make sure you get a complete head and neck cancer screening at your dentist every 6 months.
Geographical tongue? If there is no pain or other discomfort, it can be geographical tongue. Which is benign condition.