Casio Watch Battery

I have a watch problem. It’s odd, sometimes the times shows further, and sometimes shows slower.?
This is a normal Casio analog watch, with a word ‘Quarts’ below it, a silver colored, medium sized which I’ve been bought around a year ago. Until now, it shows me up this weird symptoms where it sometimes faster and sometimes slower than the current time. Let’s say now the time is 8.40 pm, and the watch shows 9.40 pm. When I corrected it back to 8.40 pm, suddenly when I woke up the day after it shows back one hour late than the current time I saw on that day, like 7.30 am where the correct time should be 8.30 am.
What’s wrong? Could it be a battery problem?
The watch is just an analog watch with no date displays.
I never opened the case or changing its battery before.
It’s just weird. What’s wrong??
i remember many many moons ago, the mechanisms in my body, would mess my watch up.. but yes it could very well be the battery : )
Casio F-91W battery replacement