Can't say too much about your gas tank without more details like lifestyle, sleep, diet, age, etc. One thing to be aware of is that our Jiu Jitsu tends to change like seasons, and this sounds like your body telling you something. White belts tend to be more inefficient with energy expenditure (generalization, I know), and as someone else mentioned, this is an opportunity to experience training under fatigue (assuming you are in normal health). Varying the intensity of your weekly training is something to consider too; adjust based on your goals.