Hi Jessy, Welcome. Yeah 2 litres is pretty small. A lot of breeders do use small containers but they have the experience manage water quality and detect problems early. I usually use small containers when I'm breeding but my current spawn doesn't have many males and I managed to fit them all into slightly larger 6 L containers. This allows me to have more time in between water changes. What I'm saying is that the shop assistant didn't give you the full story (they may not have known better).
I suspect your female is cold. Small containers generally don't have room for a heater. So unless you're keeping it in a warm room (24-26 deg C), this is likely to be the problem. Even in summer, the overnight temperatures could be a lot lower than ideal. You should check her carefully to make sure she hasn't caught velvet. This will look like a fine yellowish-gold dust. It can be difficult to see depending on the colour of the fish.
My recommendation is that you get a larger tank, say around the 20 L mark. This will have room for a heater and filter, and you can even add a couple of companion fish. But don't add any other fish until you are happy this one has settled down.
Good luck and let us know how it goes.