I had one summer before high school where I had five sports going at once:
Two basketball teams
I was talented enough that coaches were okay with me missing many practices and mainly showing up for games/meets. This meant I was nearly always running at full tilt (did I mention I was a catcher, playing every inning for both teams?) Towards the end of the summer, I got extremely fatigued, sleeping 12-14 hours every day (got the full workups for mono and valley fever nothing came back positive), a mid-back pain that I still have to this day, and I basically had to quit all the sports at once. If there was ever overtraining, I was in it.
Take proper rest or the body will make you take proper rest. Ignore it further and bad things happen.