Im confused about my mum ?

I'm 13 and one time me and my mum and my bro and sis were out shoppin and my mum was buting loads of pink lacy and other types of underwear. Then my mum went out a few days later and put everything out on her bed and I felt sick at what I saw. A frilly bra with a hole in the front and nickers with zips down the front that actually open ! It was so digusting. Then a few days later I lost my phone and asked to use my mums phone to call it. I realised there was a password on her texts/calls. She unlocked it for me and when I'd finished using my phone I saw sexting messages. Now she's told me she has a boyfriend and I'm happy for her but why does she need to do all this ? Please help I don't know what to do ?