Constipation?

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DD is almost 3 and recently potty trained. She has always been a bit prone to constipation and had a particularly bad bout about 18 months ago which resulted in a couple of tears :( She has never been comfortable pooing and often appeared in pain or uncomfortable when doing so. She would also hold it for some time before going.
Since potty training she has never done a poo in the potty. She used to ask for nappies or poo her knickers. She now seems to be withholding more than ever, often seeming to need to go for 2 days or more until she goes. Often I give her lactulose to make her go (but not too often as I'm conscious of making her reliant on it). The last few days she has been complaining of stomach ache and crossing her legs as if she is withholding. When she has her night time nappy on eventually she will do an almighty fart which results in a generally runny poo. The runninness varies from complete liquid with bits of food in it, to squishy and smooth, to lumpy. It hasn't been fully formed since last week when her nursery worker said she did a very big very hard stool.
She drinks plenty (weak squash as she wouldn't drink enough water which was contributing to constipation), eats some veg although as with most toddlers this is limited so we are trying to improve this. She eats plenty of fruit.
A while ago we spoke to the GP about her general constipation and they prescribed Movicol, but I was reluctant to try it as I'm not sure I could get her to drink it.
I'm slightly worried that all the runny poo could be a sign of bad constipation. I think it's probably linked to withholding as she rarely goes before she has her nighttime nappy on, and can then go 2 or 3 times in quick succession. It's affected her sleep as she can't get to sleep whilst she needs to go :(
I'm not sure whether to try the Movicol. Has anybody experienced similar? Any tips for getting a toddler to drink it? She doesn't respond well to bribery. Also I've heard they can't really control when they go so if we use it should we put her back into nappies full time for a while??
Apologies for the epic post, hope it makes sense as I also have a tiny baby so a bit knackered! :)
I am going to take her to the GP again this week so they can check her over, but just wondered if anyone had advice in the meantime. Thanks!

Your GP will, if they know what they're talking about, advise movicol/laxido. The longer the constipation is left untreated, then it is likely the higher the dose that will be needed to get things moving. So start trying now. In any non-boiling liquid. Liquids with some texture to them hide it better than water/juice. Fridge cold tests better than room temperature. If they can use a straw, that makes it taste less.

She had another explosive runny accident at nursery today and another at dinner so we've started the Movicol tonight. Surprisingly it went down okay. She's since done another tiny poo (more of a runny shart, sorry if TMI!!) since going to bed. Is that the Movicol working? How should I expect it to work, will her poo become more solid?? Really want to get this sorted as these night time explosions are pushing her bedtime back until nearly 10pm :(

I think the advice is to give movicol for about six months, to let the bowel heal from any weird shapes it has been stretched into by compacted faeces.

On average, the poo get runnier, then runny with lumps in it, then just runny, if they're having a proper clear out. Then the movicol is reduced to the amount that makes the poos be daily pain free squidginess. But for the clear out you're actually aiming for days of bucketloads of watery stuff, to clear the backlog, then keep it moving, so the overstretched colon can start to heal up.

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