I’m fasting 16:8 but having the same problem. I read that it’s better to ease back into eating after a fast with something small. Eating a full, heavy meal, as I have been doing, may be the culprit.
The meals I’m eating (keto) are typical for my pre-fasting life, but now I’m getting full faster, and not stopping when I’m just satisfied, and heading to the bathroom about 45 minutes later.
I’m going to experiment with controlling my portion size; only eating to satisfaction. For me it’s a mental thing. I feel like I haven’t eaten in a while and feel like I’m making up for the missing food.
Today I started with one Brazil nut. I was OK for an hour. Then one olive. I was OK for another hour. (This sounds weird, but I was out on my walk, then grocery shopping.) In the car on the way home I ate 3 slices of bologna. Fine still. Then a bit later when I got home I had a bowl of leftover roast beef, broccoli, mushrooms in a cream sauce with 2 eggs on top. I probably ate about a 1 cup of food, maybe 1 1/2 cups. After eating I was instantly exhausted (another story), took a nap, and woke up and had to run to the bathroom. 
This article is about people having trouble after Ramadan, but the outcome is the same.
A YouTube video byDy Ann Parham I watched talked about this being a transient stage as your digestive system adjusts to keto, but I’ve been keto for a couple months. Are you new to keto or IF? I’m only 3 days in on IF.