My girlfriend and another guy?

So, me and my girlfriend have been together for about 8 months, and are both 18 years old.

We have a very strong relationship, talk everyday, both each other's first boy/girlfriend, and we love each other completely.

So, my dilemma is:

She has received texts from a guy who I've never met, and she's never mentioned him EVER, in any conversations. She never brought up his name at all. I could see him texting her because I usually hold her phone in my pocket because her pockets are too small and she hates taking her purse. I felt her phone vibrate in my pocket once when we were together and I took it out for her to check, and I noticed it was this guy again.

I brushed it off as nothing more than just a friend, as I didn't want to come off as too suspicious and controlling. However, this continued on and on and I just got more irritated, as she STILL didn't mention anything about him.

About last week, they were texting apparently and he called her out of nowhere, and she quickly grabbed the phone and put it on Silent. Since then, I've been very worried and suspicious so I decided to ask her about it. I asked her who he is, and why she's never mentioned him before.

She said, "He's just a friend who I lost touch with and I never found a reason to talk about him. I didn't think it was a big deal for me to be texting him since I know he's only a friend".

I trust her and believe that he's just a friend, but I just don't understand why she decided it was perfectly normal to not mention him at all.

So anyways, she tells me today (after I've asked her about who he is and everything) and she says' "Oh, well, me and Alan are going to lunch on Wednesday". I was like "Okay, sure". I thought it was odd how she never told me anything about him but all of a sudden, she makes plans to see him.

I imagine if I never asked her about him, what would she have told me she was doing when she goes to see him?

MY QUESTION IS: SHOULD I BE ALARMED AT THE SITUATION? SHOULD I BE WORRIED?

I'm sorry if it's long, but I just thought you'd be able to understand it more clearly.

****I have asked this question before, but I resubmitted it to get a second opinion. I haven't gotten many responses, and I'd like to hear more opinions.