Hi, and welcome to TB! You've come to the right place.
What is your budgie's diet? If she is only on seeds, then I think she is overweight.
Budgies who are overweight look very "fluffy", and when sitting down, have a crease near the bottom of their belly that looks a little more "cleavage like".
With the runny feces, it could be a number of things. If she isn't going through a moult at the moment or eating lots of veggies, she could be stressed due to a moult, her diet/not getting enough excercise, or actually be in the first stages of something more serious, like fatty liver disease, common in budgies.
Don't worry though, because there are many easy solutions. I know because my Mallorn had the same problem as your little one (big, tummy crease, runny feces) and now she is fine. I changed her diet over to only pellets for about a month, and slowly added seeds back in. Now she is at a stable diet of 60% pellets, 10% seeds, and lots of veggies.
If what I described seems correct, I can recommend a few things to do while you determine the cause for sure, just to be safe.
With Mallorn, I made sure she always had access to veggies. I would suggest dark leafy greens like kale or romaine lettuce, as well as carrots, since they are good for the bird's system and also digestible.
Second, I dusted her seed mixture will a little bit of pure tumeric powder (it's a spice) for a week, since tumeric is known to have healing properties towards the liver, which is the most common organ affected by being overweight. I've hear milk thistle also works wonders as a holistic remedy, but I don't have experience personally.
Finally, I would switch her over to pellets if she isn't already. I recommend either Harrison's, Roudybush, or ZuPreem as they are all excellent brands.
Personally, I have had great success with ZuPreem fruity for extra small birds (canary and finch size) Because it is the same as the budgie blend, I got this one for Mallorn because the pellets are similar in size to seeds, which helped over with the process.
Switching a bird over to pellets can take forever and it can be really frustrating. It must be a gradual process so the bird gets used to it. I'd be happy to give you some advice on switching a bird onto pellets if needed!
And of course, a trip to your avian vet might help to pin down exactly what you can do to help your little girl!
I hope this helps