Is my mom cheating on my dad?

I don't know when it started, (probably about a year ago) and my mom started to have long conversations on the phone.
The weird thing about it was, that she didn't mind for us to know that she was on the phone,but when we asked her who it was she...

wouldn't reply and sometimes even got angry and said it was none of our business.
Me and my two little brothers made fun of her that she has a boyfriend.
She just laughed, but then got very angry.
When we ask her who it is,she says that it is her friend from work. Though it's very strange that she talked to him, when we were on holidays inFrance, Greece and also at some random times. Also when she's in a bad mood and she talks to the 'someone' on the phone her mood suspiciously gets better.
Recently on the holidays in Greece she borrowed me her phone and accidently I read her messages (because I gave her number to my friends as I didnt have my mobile with me, and I was checking my own messages) and I found some very strange ones...
I know I shouldnt have copied them,but this is what some of them said:
'Are you enjoying the brillliant light of Greece that illuminates even the heart of stone? Is the bellowing fragrance of the mountain herbs stilling your soul?Does time seem suspended ? Then you have been captured and you will never want to escape. I'm desperatly unhappy not to be sharing it with you. MU WU LU XU LB'

'...Fires are in news.PLEASE keep away from danger!
W2XU because LU & MU LB'

'...The Greek fire is a calamity. I hope you are all ok and able to get on with holiday. Worrying about you! LB'

and this is the messages I found that my mom had send him:

'Mycanae. Roman (my dad) getting suspicious why I spend lots of time at Tomb of Klytemnestra. (Klytemnestra was a Greek queen,who with her lover murdered her husband !!! Coincidence??) Also visited Corinth and Epidaurus. Now moving to Petalidi near Kalamata to explore the Pelopennese (Sparti,Olimipia etc.) It's so touching and emotional. I am missing you more and more. Thinking of you all the time.
MU WU LU XU LK'

'We are safe at Petalidi. The road to Tripoli is closed and there is also a big fire near Olimpia. Now going to Pilos,Nestor's Palace. Police keeping us informed,comming twice daily to camp, we are able to contact our family twice daily free. Enjoy Malcolm's anniversary.
MU WU XU LU LK'

After I found this messages I showed them to my brother and we started figuring out what : MU WU LU XU, could mean and we figured out that it's probably: Miss U, Want U, Love U, Kiss U.
If it's not that then we dont know.
Also I told my dad that I found weird messages on moms phone and I told him what I suspected and he was actually angry with me and said that it was none of my business to look at my moms phone.

Also when we joked about my mom having a boyfriend she said that my dad could read all of her messages right now if he wanted, and that was the day when I found the messages so the messages must have been there, cause some of them were more the three days old.

Also why would be my mom so stupid not to delete the messages of her phone ??

Please,please help!!

I really dont know. But I think that you should definatly talk to your mom about this. If she is getting upset when you ask about it then maybe you should still have a mother to daughter talk. Make her feel like her secret is safe with you. If she isnt telling you then it could be that she is afraid of you hating her and that you will never thnk of her in the same way. Maybe if you make her feel like that isnt true she may open up abit. But dont forget that if she still wont tell you. Its possible nothings going on at all. So make sure not to push it! theres nothing you can do if she says it isnt true.
Hope this helped