Hi Layla85
It sounds like a skin tag to me, although I'm no expert of course. Do you have a CRS who is treating you at the moment? If you're worried you could ask them to take a look just to give you peace of mind.
I have a skin tag, its a flesh coloured lump that looks like it is blocking my arse too. It's actually attached just inside of the posterior and it moves aside with a BM. To the side of this I have another softer bump thing which isn't as hard as the skin tag and can cause sharp pain at times. The CRS said it may have been a hemmie at some point but nothing to worry about now.
Hemmies are not usually flesh coloured, more purpleish/blueish from what I've read. Apparently they can also settle on their own. I don't know how true this is. I doubt movicol could cause it as it is most likely that a form of trauma to the anus that has caused this. I've just checked the side effects of movicol and its doesn't state anything like this.
http://www.netdoctor.co.uk/medicines/100001760.html I dont take it myself so I can't comment from personal experience I'm afriaid. I take a different osmotic laxative called lactulose.
Also, just a thought as you mentioned the anoheal, I wonder if you may be having a bit of sensitivity to this. Perhaps try applying it away from this bit (you may not be applying this anywhere near that so it may not apply) that hurts for a couple of days to give it a chance to settle.