Platelet counts can drop for a lot of people when they are sick, but most just never know it because they don't have any symptoms. The counts can rebound on their own. In a person with ITP, or the disposition to acquire ITP, counts can drop every time the person gets sick. Some people find that counts go up when they are sick. It's different for everyone.
In children, they sometimes have acute ITP, meaning that counts will drop due to an illness, but they usually recover when the illness goes away or a few months after that. It seems like your daughter either has a recurring viral infection, or just keeps catching one thing after another. Tis the season. I don't think at this point that you need to worry about something other than ITP. Sometimes medications, like antibiotics, can cause a drop in counts too, so that could have something to do with it.
The good news is that your daughter responds to treatments, so in that case, she really isn't in any danger since you can get her low counts up fairly easily. You are doing all that you can do by monitoring her and treating when necessary. Hopefully, this will pass soon. If not, be assured that ITP is manageable.