My daughter has found that certain iron infusions are easier to tolerate than others. The first two times she had infusions, she was given a certain formulation of iron that was infused over an hour and had absolutely no side effects. Last time she was given one that is infused in 8-10 minutes and that one caused nausea and constipation for almost a week! I can't remember either of the names unfortunately. But my point is that there isn't just one type of iron infusion - there are many different kinds. And according to my daughter, she felt MUCH worse on iron pills and liquid iron than with either type of iron infusion.
My daughter was also exhausted and very pale when we found out she had low Ferritin the first time. But after each infusion, she felt better and better!! She didn't have any side effects at all the first two times. The first time her Ferritin dropped to 3 and the second time and third time, it was 8 or 9.
I wouldn't panic since you have LOTS of medication options left for UC. MTX, Azathioprine/6MP, Remicade, Humira, Stelara, Entyvio, Simponi and even Xeljanz.