By mistake, the positive and negative terminal of a 9v rechargeable battery were connected to each other by me, and when I noticed it, I quickly removed it and the battery became hot a little. Did it caused any harm to my battery, Or should I use the battery or is it damaged now? Will it explode if I put it to charging now?
Although one cannot be certain when treating a battery to such abuse, it is likely that the battery will be safe to charge and to use.
The Uniross 210 rechargeable battery has NiMH (Nickel metal Hydride) chemistry.
Shorting the battery is a very bad idea and it may have damaged it somewhat but if the short circuit was not of long duration (seconds rather than minutes) the damage may not be too severe.
I have occasionally accidentally shorted NiMH AA cells for short periods in the past with substantial overheating. These have far higher discharge currents compared to the 9V battery and they were still usable after this treatment.