I was playing a custom game on Summoner's Rift as Nidalee (default skin) and a blue rectangular box appeared above my head after I used Primal Surge. I haven't tried to reproduce this but I will give all of the details I can about what happened. It happened after I used Primal Surge. I was level 18 therefor all of my skills were level 5 and 3 for ultimate. I just got a double kill off of I believe a Warwick bot and a Fiddlesticks bot. The first kill was Fiddlesticks. I was not under the effect of any spells. But I did have Red and Blue buffs and all of the Elixers and Oracle's. My items were as follows: Mejai's Soulstealer, Sorcerer's Shoes, Archangel's Staff, Rabadon's Deathcap, Lich Bane and Athene's Unholy Grail. In that order. I had 16 stacks of Mejai's and the kill on Warwick put me at 18 stacks. It was about 28 minutes into the match and I was by the stairs at the top right of the map leading into the enemy base. I had roughly 900 AP and 39 + 10% spell penetration. I had killed Warwick in Cougar Aspect I believe then hopped away with Pounce, transformed and immediately healed using Primal Surge. Oh and I think the Archangel's was currently giving me ~300 mana from the passive stack bonus. Also (I very well could've been hearing things. Or this didn't happen) I could have sworn I heard the sound for Soraka's Infuse ability right as the box appeared. I did have a Soraka on my team as well but she was nowhere near me.
This is all of the information I can think of. Yes it is lengthy but I recall reading a Red post that no information should be ruled out as unnecessary and this was everything I could think of possible that was going on at the time with me and the bots involved directly before the incident occurred. If I could have some Red insight on what of this is pertinent so as to expedite my bug report writing it would be much appreciated.