Signs of heat

What are the telltale signs that she is in heat? I was planning to wait until after I move to have her spayed, but I fear that mother nature may have gotten ahead of me. Last night she was walking around in the middle of the night lightly meowing (not howling, but repeatedly and loud enough to keep me up). I went to check on her and she was fine. I brought her back to her bed. However, she got up again later and was doing it again. This morning she totally normal.

How old is she?

What you are describing does sound like the beginnings of going into heat, and the time of year is right.