Shall I tell my best friend im kinda talking to someone she dated?

Me and my best friend are very close. We have been best friends for a while and we have never done each other wrong at all. My best friend has a boyfriend now who she is very serious with, talking about marriage etc. We both worked at the same place and met this guy, I spoken to him first at one point but we drifted then she spoken to him after me which I allowed anyways I didn’t mind. Them two spoken for about 2 months and she did like him, she ended him though since she found someone else. So anyways me and this boy kissed and I didn’t hesitate to tell her and she was a bit angry with me. She told me if she wasn’t single she’d be mad but since she is in a relationship she’s not as mad but she still was a bit angry. This was 2 months ago, ever since me and him have spoken everyday. It’s so bad I know it only hit me the other day when I ignored his text and he double texted me which kinda made me realise we do speak a lot. She is very happy in a relationship but I’m scared to tell her me and this boy she dated are kinda speaking. But everytime I’m with her I can’t use my phone incase he texts and it’s horrible hiding stuff from her, how can I tell her? I don’t wanna hurt her she’s my my best friend. She’s been in this position before where one of her friends dated her ex boyfriend execpt they were together for 4 years so it was a bigger deal and she forgived her and became very close. Shall I tell her? Me and this guy aren’t that serious yet though but I’m starting to realise that we talk a lottt
Shall I tell my best friend im kinda talking to someone she dated?
Post Opinion