Why are my trays feeling tighter at the end of 2 weeks?

I'm almost at 2 weeks wearing my first set of aligners and over the past couple days it feels like my trays are getting tighter. The bottom trays is really difficult to remove now. At the back it pops right out the it's very tight on my front teeth. I wear my trays about 21 hrs/day on average. Should I keep wearing this set until the bottom tray loosens up at the front?