Hard to say. It's difficult to know without having more information, & particularly without being able to see the spots. Probably the best course is to see a dermatologist. They will get history about how long this has been going on, how long they last, any related symptoms such as pain or itching, anything that seems to cause them to flare up, & then examine the area. This will help determine the cause and tx.
Hard without pic. Unfortunately, it's really hard to comment without a picture, but sometimes people get telengiactasias - little superficial red blood vessels. Acne can look like little red marks (not always pimples). It can be many things. Make sure you get it checked out by your doc!