Some ways you can tell if she is comfortable and relaxed is if she flops down to rest, lays her head down with eyes closed while you pet her, or if she seems to be interested in you and what you're doing. If she is stressed she'll have skittish abrupt movements, eyes will be very wide open and her body will be tense, or even shut tight and not moving around, she will probably huddle at the back of the cage, and when picking up and holding her she may shake from fear. I had a rabbit that got seriously ill from the stress of being in a new home, so if you notice any fear behaviors, it would be good to try and do things to minimize her stress as much as possible.
But if she seems comfortable and relaxed around you, then the soft poop is most likely due to the new foods, and should clear up in the next couple days now that you've stopped them. If you had to switch her to a new brand of pellets, that could also have something to do with it. Any new foods(besides grass hay) need to be introduced slowly into a rabbits diet, or they can possibly develop digestive problems. So if the soft poop isn't cleared up in a few days, try reducing the pellet amount until the soft poop stops, then you can gradually start increasing it to a normal amount. But you'll want to try and figure out how much of the pellets she is eating in a day. An adult rabbit shouldn't have unlimited pellets. 1/4-1/2 cup would be normal for a 5 lb rabbit. But if you find that she is just eating mostly pellets and not much hay, you can decrease the amount of pellets you are feeding and that should get her eating more hay. She should be eating a pile of hay per day, that is about the same size as her body, but you also want to be feeding her enough pellets that she stays at a good body weight. If she is under 6 mo. old, then her diet would be slightly different. But feeding a measured amount of pellets per day is also a good way to know if something is wrong with your rabbit, as they will usually stop eating their pellets first.
Hopefully this will get cleared up quickly. I know how stressful it can be having a new rabbit that might be sick.