Why did he lie ?

To the point. Short and sweet. ..maybe.
A little about us and me.
My boyfriend n I have been together for almost 4 years. We have a 16 months old baby. I'm 5 years older than he is and he is in his late 20s . Me early 30s not that it matters but just throwing that out there
Long story short and you can let me know if you have any questions because I suck at telling stories but he lied to me about his ex girlfriend. She just so happens to be good friends with one of his family members which is totally fine ! If he didn't lie about me being around her when I asked him 3 days after this event that we were at and he said no she doesn't even matter which I'm sure she doesn't but it gets deep because this situation could have been avoided if he just brought it up to my attention at the time and I didn't have to find out from his sister that I met her! Listen I'm not going to lie and I know a lot of people might think I'm just being insecure and I need to get over it because this other Chik is married happily but the thing is. My thing and thought is. He lied because not only is she like 8 or 10 years younger than me she is extremely pretty and that's the reason I think he lied. I say it gets deep because me being nosey and insecure went thru his phone checked his Google search information and found that on numerous occasions has searched her several times over the past few months and then lied about that too. This relationship ended a long tI me ago and I'm not fearing he will leave me for her or she will want him but why did he lie. How come he denies this. Why does he even bother checking her out. And why can't I get this gut feeling out of me that I'm not good enough for him and in his thoughts the woman he thinks about and really wants is someone as beautiful and fun as her. And it's her that he wants. I'll never be her is what I'm thinking. I'm not thin at all I'm a thick kinda overweight still have my baby fat from our child which is my fault and I know I need to work on it but I just hate feeling like This. I ask my friends they just tell me what I want to hear but I need to know. If I'm right about him not fully loving me. Comparing me and wishing he had her instead. On top of all that. Last thing I promise. He hasn't asked or said anything about marrying me. Ugh. My life. My feelings. Why? And yes trust me I did not forget our baby. Best thing that's ever happens to us. She is always number one I just need some advice.