Is this considered cheating?

[KollegahDerBoss]

Backstory:
My girlfriend has been a little too jealous lately, because she saw me talking with some genuine girlfriends over the internet.
She caused drama but I ended up talking it out with her and understanding her.
She also told me "I rarely talk with other guys besides you."
The thing is - she now seems to want to take revenge because of that what happened.

Keypoint:
My ex-workmate sent me a photo of how my girlfriend is flirting with him and sending him heart smileys over Whatsapp. (He doesn't know we're together)
How should I react to that? It is indeed an indirect way of cheating - and an act of revenge - how should I respond to that?

 

[KollegahDerBoss]

I solved the situation:

I told her I found out - she then wrote a 10000 words text to explain to me why she did that - what she felt for the guy (nothing), etc.
Anyways, she was too insecure because I gave her the feeling that I didn't love and value her.

I explained to her that she doesn't need to do those actions - but actually tell me - and reassured that I do love her - but still left that bit of a challenge which keeps her intrigued