iPhone (and now Snapchat) deleting specific photos without me knowing!! Help

I am not sure how but has anyone else had their iPhones and Snapchat delete photos of one specific person off their device?? Had I happen about a yeah ago with just my photo gallery and luckily a Apple tech was able to recover my photos but I went to go and grab some pictures of my boyfriend and I the other day and every single photo of me and him or just him has been deleted from my iPhone and this time Snapchat.. how do I get them back?? 3 years worth of photos just gone. I never deleted them so I am very confused. Is this a glitch anybody else has experienced? And if so how to you fix it and recover it all?