Must be water retention indeed.
The cause of this could be the testosterone. Many guys have a phase where water retention occurs for a few months. This tends to happen after a few months to half a year in. You're a bit early, but it is possible.
Water retention can also happen due to other medical reasons, mainly related to the heart and blood pressure. This is unlikely in your case, but you did just start testosterone, which can cause issues with blood pressure. The best course of action is to have it checked out, especially if it gets worse.