What Causes A Lightheaded Feeling?

Hello. For the last 2 days, I have gotten lightheaded when laying down intermittently. It's a very uneasy, bad feeling. Also, I have RA and take injections of Humira every two weeks to help keep that under control.

Aside from the lightheadedness, I'm not experiencing any other symptoms.

I did take my Humira injection yesterday and last night was the first time I experienced the lightheadedness. I experienced again this afternoon about 4pm when I laid down to rest.

Would really appreciate your thoughts,
Kim