I'm glad you already have plans to see a doctor
Until your appointment, I would suggest taking a multivitamin on a daily basis and re-evaluating your diet. Is the food just low calorie (like pre-packaged meals or 100-cal snacks only?) or is it low calorie AND nutritious (lots of veggies)?
Make sure the multivitamin you take will have enough B12 in it. If it doesn't, consider taking a separate B12 vitamin until things get figured out.
Good luck!