Welcome @lorrism124, It's the pits when a medication or treatment causes a weight gain. There is a study about Prolia causing an unintential weight gain -- Prolia and Weight gain - unintentional - a phase IV clinical study of FDA data: https://www.ehealthme.com/ds/prolia/weight-gain-unintentional/

Here is some more related information that might explain what is happening --- "No, weight gain wasn't reported as a side effect of Prolia in clinical studies. But weight gain may be a symptom of a side effect called peripheral edema. With ..." --- Prolia side effects: What you should know: https://www.medicalnewstoday.com/articles/drugs-prolia-side-effects

You mentioned gaining 5lbs and working out 3 to 5 days at the gym. I have an inexpensive home bathroom scale that measures your weight and shows body fat, muscle mass, water weight, and BMI. Do you have a bathroom scale that would show if it's the water weight that is increasing?