How do I respond?

A co worker and I sorta have this thing at work but I'm trying to not communicate with him due to I believe he has a girlfriend and has kids with her. He normally says bye to me before he leaves but I haven't been respondent to him lately

He texted me this

Like whats going on with you? i feel sum type of way one min u all cool, we're kissing and then the next min u acting distant short answered an weird like im bothering u or something let me know if i need to put the fall back game in full effect

How do I respond?
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