I have the correct explanation. It is correct, I don't just believe it is correct. If it is incorrect, there's a hole in the plot or I missed something that I've searched for for a week. It's pretty obvious, actually. I don't want to spoil anything, though... :/
But still I do. He tries the sedative in Yusuf's basement. The strong one that sends you to limbo if you die when it's used. Cobb dies in the dream on the rails. He "wakes up". What makes it EXTRA obvious is that when he tries to spin the spinning top, he is interrupted and he can't spin it. We never see the spinning top fall again. He is dreaming by the end. Limbo starts in Mombasa and Mombasa and outwards is a dream.

Can anyone prove me wrong?

I figured it out myself