P0128/ coolest revoir issue?

So, I changed my coolant temp sensor
yesterday. A lot for coolant came out.o
refilled the reservoir, and when I rode
the car the gauge was fluctuating
between normal and over heating (well
almost. It was 2 bars under the
overheating) for a while fluctuating
every other few minutes.....when I hit the
freeway to go home the gauge
stabilized and remained Midway. It
stayed the same this morning stayed
on the middle....but the coolant
reservoir was a bit empty is this
normal? Or did I mess something up.
The car drives fine, not over heating,
not check engine lights.