with rapid weight gain like this in a situation like yours, a *majority* of the time it is 1 or a combination of 1-3 things.
1. Rebuilding of lost, but previously held muscle tissue. Even if losing fat at the same time can cause scale weight increase
2. Water retention from several things, cell swelling as a response to training, increased muscle glycogen if carbs are moderate to high, and sodium if it is high. The drugs may play a role here, though I don't know enough about them to say that is in play.
3. Eating too many calories. You say you are losing fat, okay, but picking a target number of calories, even for a recomp, is a bit of trial and error. You may be eating too much. That would be something you and your coach need to huddle up about and make sure you don't need an adjustment. There is also the possibility of poor tracking of intake. Are you eyeballing serving sizes or are you measuring?
- Are you eating at maintenance for the recomp or targeting a deficit?
- How confident are you in your ability to accurately track your intake?
Scale weight is only one indicator of progress/change. This is why you need to be taking photos, the same time of day, same place, lighting, etc. for consistency, and also do some simple basic measurements. Arms, Chest, Waist, Thigh. This can help flesh out what's going on. i.e arms, chest, and thigh increasing and waist decreasing, and clothes fitting differently, but scale weight increasing indicates muscle being rebuilt and fat being lost.