This is my 6month old daughter , I have no idea what happen to her eye it started yesterday morning . Me and her dad…

This is my 6month old daughter , I have no idea what happen to her eye it started yesterday morning . Me and her dad were both laying next to her when he pulled the cover over her and pulled it back she started crying like something seriously hurt her, I'm so upset because I have no clue what this could be , I always wash all pillows and covers, vacuumed the floor dusted everything , it didn't give her any pain but it did cry really bad and swell up . I took her to the nearest urgent care and they said it was allergies!! I seriously didn't believe that, but then this morning when we woke up, it was fine. Does anybody have an explanation for this ????