First and foremost, it looks like you’ve deliberately gone off sick to spite your boss.
If what you’ve done is fake that you are stressed is totally wrong and using a condition to your advantage just to get time off and a holiday. Especially when there are genuine people who are suffering from genuine full on stress, which makes them seriously unwell.
Clearly your boss knows full well why you’ve gone off sick, because you didn’t get your OT paid. Surely, if you knew you’d be working extra hours, you’d of confronted your boss first and asked what the terms of doing the OT was, if it was paid or unpaid. Assuming it wad paid
OT isn’t the right thing to of done, this needs to be agreed.
I’m not saying this is 100%, but this could potentially mess up your current position, cause you to be treated differently now, by the way of being untrustworthy, a snide and unreliable. Your boss is obviously unhappy about this and right to be giving you a written warning. He doesn’t need employees like that in his firm.
Also, this could mess up future employment as you may be refused a reference from your boss, on the basis you faked the stress to get time off and wasn’t honest.
This was a bad move. I would arrange a meeting with your boss, be honest with them and tell them how you are feeling and what you can do to make things right.
Maybe next time you choose to do OT, if he even bothers to ask you again, then maybe discuss the terms before agreeing to it.
If HR have also agreed your boss was in the right, then you really do need to use this situation a learning curve and move on. Don’t get on the wrong side of your boss, other colleagues and the company. Don’t take time off for stress and then book holidays as it looks totally fake. Ok your stress may be genuine, but this really isn’t how it looks from a bosses point of view.
Best of luck! Maybe some good old fashioned sucking up and a written apology to your boss
might be what’s needed here. 🫣