If you submitted your doctor's certificates then you certainly shouldn't be taking this as annual leave - it's hardly holiday, and any employer worth their salt should want you to be signed off sick so allowances could be made when you are fit to come back. Were you signed off as fit to return, by your GP or hospital.
I don't think it's your sickness in question, it's the way you managed the reporting sick and coming back. You need doctors' notes.
Re the separate training issue - you can certainly be disciplined for failing to meet the job description, or capability. However before a disciplinary, at my place you have to have a capability meeting first so everyone can do their best to raise performance, e.g. by training or by working differently - disciplinary comes a lot later if you are failing to come up to scratch.
BTW you have to be notified of disciplinary meetings in writing, not only verbally - so you have a chance to bring a union rep or personal representative with you to the meeting.