am i going mad

I've been with my partner now nearly 8 years and we have a 4 year old daughter. the other night I saw my partner messaging a girl who he used to work with on Facebook messenger this went on for half hour. when he went in shower a message pop up with her saying 'lol really' he deleted the whole conversation he had with her so I asked him why he was messaging her and his reply was she wants her old job back and that nothing going on if that's all it was why delete everything don't understand it then the next day I noticed he had her number saved in his phone again I asked him why he's got her number and he said she gave it me to give work so the boss can phone her for her job back. all of it seems dodgy started to think he's up to something