As far as I know your "experience points" are your total score. osu! wiki says following of this total score:
The total score is the
total sum of all scores on ranked and/or approved beatmaps, including failed trials. If a player plays a ranked song for the first time and gets 500k, both ranked and total score increase by 500k. Then, the player plays again and gets 400k, only the total score increases by 400k.
So you pretty much get it from everything what you are doing in osu! as long as you are playing beatmaps.
It also says following of this level system:
The level of a player is based solely on the player's total score. The total score requirement for a given level is as follows:
5,000 / 3 * (4n^3 - 3n^2 - n) + 1.25 * 1.8^(n - 60), where n <= 100
26,931,190,829 + 100,000,000,000 * (n - 100), where n >= 101It might be better that you go to read it yourself from the
osu! wiki.
But to answer to your actual question I don't know why that happens. Have it ever happened before? I get it from every beatmap. Maybe your connection is not working properly? Have you tried to booting your internet connection? Or trying to restart osu! again? Have you tried different beatmaps? osu! was victim of DDoS attack recently it might have something to do with it. Sometimes osu! is not working correctly if you start it as administrator so if you have been doing that you should try to start it with not as administrator. Nothing else comes to my mind at this moment which could maybe fix this.
EDIT
It also seems Bancho is down so it might be reason to that.