I'm not a doctor, so I can't give any valid medical opinions.

But from the POV of someone who's dealt with multiple medical issues, my advice is to try to assume it's minor unless/until tests indicate otherwise. TV medical dramas notwithstanding, the most catastrophic possible cause is not usually the actual cause.

B-12 deficiencies can cause weakness and fatigue as well. Luckily, that's pretty easy to correct. Dietary sources are the best in terms of bioavailability, but supplements work too. In some cases, they can even do B-12 shots, if your levels are really low or your body isn't processing it properly from dietary sources.

If you're worried about the birth control, I'd recommend talking to your doctor about stopping it or switching to a different method, depending on your circumstances. But I'd recommend only changing one thing at a time so that if your symptoms abate, you know what thing helped. I'd say start by addressing the B-12 deficiency, and if that doesn't help, then maybe change the birth control. But basically, do whatever your doctor recommends--they know better than me.