Should I be worried?

I've had an IUD for about 4 months now and my periods have been very irregular. Either too long or extra spotting. I had sex the day after my period stopped last month and then again in the last 2 weeks, I've been under unbelievable stress and my period might be "late" according to the app. When should I start to worry if at all? I don't think I'm pregnant but you never know