Pearl Danio Acting Very Strange

I am new to fish keeping and have only had my 125 litre tank about 2 months. I am keeping a few different species of danios and all has been well until today when I spotted one of the pearl danios spinning round and around then it stopped at the bottom of the tank very still for about 5 minutes. I though it was on its way out but then it started swimming normally again. About an hour later it started darting around the tank then stopped at the bottom again. I thought it might need isolating so put in on its own in a separate small (48litre) tank for a few hours. But it seemed so stressed darting around that I have put it back in the big tank again overnight. (the small tank was filtered and the correct temperature). I have tested the water for ammonia and nitrates and all seems fine. The other fish seem okay and since I put it back it has been swimming normally again. Can anyone suggest what I should do if it carries on acting this way? It doesn't look any different. Thanks 
Just to note - I did my usual 30% water change on Saturday. I currently have 25 small fish and according to aqadvisor I'm about 75% stocked.

Thanks for your reply Sue. Since last night the fish has been acting normally. It's very strange. The temp is 24% because I wanted to add some honey gourami so I have slowly increased the temp over a few weeks but it has been 24 degrees for about 4 weeks without any problem until yesterday. All the fish are okay again now. I can only think he ate too much blood worm and maybe had a stomach upset!
Thanks for your advice re aqadvisor.

Okay thanks Sue. I'm going away for a week on Friday so I guess I may have been feeding them up a bit too much in preparation. I had frozen blood worm in small ice cube blocks and just put one block in but there does seem to be quite a lot and they do look very full afterwards. Maybe I should cut the block in half? Anyway, The fish is till swimming okay with the other now so i think you were right - he had stuffed himself!