Good timing- I just read in the May issue of Health that dark spots on the face (including upper lip) are common in women who are either pregnant or on birth control (which includes me). The article says that the spots tend to get darker with exposure to sun. It also said that the spots usually fade with decreased exposure to sun, discontinued use of birth control, or when you are no longer pregnant... but they do not always fade. If they do not fade, apparently there are lotions that you can get at drug stores to lighten dark spots on your skin. There are also some prescription medications that can help.
I believe that it's called melasma. My dermotologist gave me a cream for it last year, but it didn't help much. When I see her again in July, I'll try for something else. I hate the way it looks. I'm also not pregnant or on the pill.