Lower Back Pain while Deadlifting

I'm currently on my second month of FB (started with FB Strong and am now doing Booty Boot Camp + FB Abs) and I've loved it so far. The only problem I have is that I get extreme lower Back Pain when doing the Deadlifts, which makes me have to pause a lot more during the workouts. I use a single 20 kg dumbbell, make sure to keep my back straight the entire time and squeeze my butt at the top. I always keep the weight near my body, so it touches my shins. I'm able to go really low with the weight because my hamstrings are very mobile and I can't do that at the Gym because their plates are bigger. I've been thinking maybe this could be the Problem? Do any of you have tips to avoid lower Back Pain?

Edited