Oh poor thing, she's got a shed load going on hasn't she? 
Hope for the best, prepare for the worst.
The runs in pigs (as in proper liquid, not just transitory soft pooh because they've eaten too much veg) is bad news.
It can be a stomach upset , yeast over taking the gut, or even a bacteria carried by wild rodents (who knew? I didn't until I googled. And when piggies graze you have no control over whats been on the grass)
Gas can be bloat- has she eaten alot of greens or grass? Some of the brassicas (cabbage, green veg like spinach,brocilli, and lush grass) can cause this.
You will need to keep her intestines flowing so yes, syringe feeding (she'll be resistant, they don't enjoy it. Make sure you put the syringe just behind the teeth, squirt a little, let her chew. Then you know it's been swallowed not gone in her windpipe )
We had Fibreplex for GP3 but it's difficult to hold in place and push the syringe while holding a piggie. It's a two person job.
We put piggie on DD lap with it's bum against her tummy then she held the muzzle while I fed.
Spaying for a sow is a big operation, the scan will be the only way to see what's happening. Has she had any other symptoms like hair loss?
18 months is young , you need to weigh up her life expectancy (can be 8+ years) her quality of life (more important than length of life IMO) and the risks of surgery.
But for now, keep her comfy, keep her fed,keep her hydrated.
My lardy 18 month old boar had soft pooh , we took him twice to the vets , he had antibiotics and saline infusions (within 8 hours of us noticing his soft pooh had turned to liquid).
Sadly all our efforts were in vain.
Give it all your best shots and crossing fingers for her and you
-clover.