Have Crohns and now employer wants me to sign some disciplinary form, please help

Hi PFC,

I have Crohns disease and have been suffering from it for a long time, with the worst of it being in the last 6 months.

A few months ago it was so bad that I had to go on medical leave. I am the only person at my job that performed my duties at the time meaning my employers had no one to cover should I go on medical leave, I should have gone 2 months earlier but I tried to stick it out and pull a solid for them and try to get better while being hammered with an ungodly amount of stressful work.

I recently returned to work and my managers blindsided me with a meeting in which they accused me of basically being a shit worker during the times in which I was dying from crohns and still trying to work for them. Essentially, me trying to pull a solid for them and stick it out got turned around into me being punished for it now.

Now they are trying to get me to sign some form acknowleding me showing up to work late (was very sick from crohns) and not performing duties up to par. I feel like if I sign it, it will give them just cause to terminate me at any time and without me being able to get on EI.

In the same disciplinary form they also make mention of my medical prescriptions and essentially telling me to not take any substances which could impair my ability to do work. I have a medical marijuana prescription, makes all the difference some days.

Personally I don't give 2 shits about consuming cannabis at work, however during the period that I was very sick and waiting for proper medication I had to consume some before work to deal with the pain. The disciplinary form makes mention of coworkers smelling marijuana on me (which personally seems bs to me since i vaporized oil). I don't think I'll need it going forward however signing this form would be saying that I consumed cannabis at work, which I did a couple times, with a prescription.

What options do I have here?

Is refusing to sign the form just cause to fire me?

Are they allowed to request I produce my cannabis prescription?

Can they fire me for consuming prescription cannabis? (I work in a non-safety sensitive role)

If I sign this form can they fire me with just cause at any time?

Please help, thank you