I woke up with a low humming sound in my
The humming sound you describe is known as tinnitus. There are various causes for tinnitus. By your description - it started while in the bed and then disappeared thereafter - the cause could perhaps be a blocked eustachian tube or benign inner ear dysfunction (related to posture).
Since you are alright now, I will not be overly worried. It may not recur again for now. Stay relaxed. Drink plenty of fluids and stay hydrated.
Hope this helps. Let me know if the problem recurs or if you develop fresh symptoms.