Received a weird photo in which my gf was doing something unreasonable for another guy

[Leoooon]

Yesterday, my girlfriend sent me a picture (is a link generated through YOSO, I didn't see that app before, is some service like Imgur) on WhatsApp, but she deleted it after a minute, claiming it was meant for someone else. I suspected that she didn't realize that I had seen it, so I pretended not to notice. 

The image was a little blurred, but it looked like she was doing bl*wjob. The photo vanished so quickly, in about a second, and I wasn't entirely sure if the person in the picture was actually my girlfriend. 

I'm planning to propose to her next month, and I genuinely love her. I am angry, upset, disappointed and sad. What am I supposed to do now? Should I ask her directly why she sent the photos in that strange way? Who is the person in the picture? Who is the original recipient? Should I reconsider my proposal plans? Should I think about ending the relationship? Or should I just ignore it and go on with my marriage plan as if nothing had happened? 

I have too many questions to ask, but I can't ask anything. 

[S2B]

I think you do need answers to those questions. Why didn’t you ask her right away?

 

[Leoooon]

18 minutes ago, S2B said:

I think you do need answers to those questions. Why didn’t you ask her right away?

 

Actually I'm too afraid to hear whether she admits that the girl in the photo is her. I think I am not able to accept it.
She would not even do that like she did in the photo for me. 

[Lisa]

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