Can An IUD Fall Out Without Noticing?

Hello im 18 years old and I got the iud put in when I was 16. Do you think it could of fallen out without me noticing? I had sex about a week ago and just started my period so I'm not sure if I should be concerned. I've lost about 20 pounds and wonder if weight loss may cause these regular periods. Where as the IUD stopped my periods to once or twice a year.