Sorry to hear you're feeling unwell. If you're in the middle of a bad flare, 5mg of pred sounds like quite a low dose. Mercaptopurine can take a long time to get into the system and have an effect. I've been on it a few months now but I'm tapering down the pred and then raising it again when I get low enough that a flare starts off again. I've just had my Mercaptopurine increased to 50mg today, and my GI's advised me not to taper my pred below 15mg (my threshold for wellness - I start to get ill below that) until the extra dose of mercap has had a chance to kick in.
If you haven't seen your doc recently (or even if you have) I'd get an appointment to discuss this.
I started on a high dose of Azathioprine (a type of mercaptopurine) and had a reaction, but I'd been really tired while I was on it. They switched me to a low dose of Mercaptopurine (started at 12.5mg) and very slowly raised this, and I haven't had the same level of fatigue.
Also, many people posting here have found that taking the Mercaptopurine in the evening can help (ie just before bed), if you're currently taking it in the morning - that way you sleep off the worst of it. I split my dose in the am and pm which seems to work ok for me.
Also, have you had bloods done recently? UC can also cause anaemia from low iron levels, which would also cause fatigue. They should be monitoring your bloods pretty closely having started you on Mercaptopurine, so it won't hurt to ask about your iron levels as well.
Hope this helps. Feel better soon.