My ex broke up with me around 3 to 4 months ago, when she told me she needed a break. It was hard for me to just lose her that quickly but we got to the point where we were almost strangers, and she had a new boyfriend (that was her ex before) So to sum it up, she broke up with him, then found me, trashed me, and went back out with Mr. Awesome.
So, now she is having problems with him again, she has begun talking to me and acting nice out of nowhere.
I've got this terrible feeling that I'm a backup, like I'm number two on her list of who she wants to be with.
She has told me she wants to be friends and doesn't want to hint at anything (like a relationship), but why would she want to talk to me about this?
It seems to me I'm going to have to man up and tell her all this, and I need to know, is that's a good idea?

Well... with the details you provided, there could be 1 of 2 things happening.

1. She likes you as a friend, trusts your judgement, and knows that she can rely on you to give her sound advice--as a really good friend would.

2. She's trailing you along as a backup--which is what you are thinking now anyway.

The best solution would be to ask her flat out what she's doing. Don't stall or give her warning, or she could think up some lame excuse or lie. Just ask her to be honest with you about her feelings toward you and what is going on. If she even starts to stumble in her response, I'd bet she's playing you so that she's got a backup... however, you know her, I don't, so you should know enough to know whether she's stumbling to think up an excuse or whether she's just truly shocked that you would ask such a question.

Either way, you probably aren't going to have a relationship with this girl again... I'm sorry. :( But, maybe that's for the best. Good luck!

You're her emotional crutch. She wants to feel needed and or cared about. The only way you will be number two is if you go back out with her. If I were in your shoes I would turn her away. You know she did leave you for this guy and she lost out on the benefit of your guidance when she left. I would cut her off mainly because it's only when they have problems that you come into the picture. If she had been trying to be friends before the problems that would be one thing but it's only when she needs to fulfill that void he clearly can't fill does she contact you. What a user. Don't be second man, tell her that when she ended it she lost the privilege of you being there for her. Take care.

Give her a wide birth and ignor her. Get some self respect and move onto something new. If you are going to put time and effort into someone give it to someone new. Then there is no history and only new experience to enjoy.
Trust me it will only end up the same way it was this time only more quick and you will be kicking yourself for putting yourself through this kind of crap again then the pain of another split with the same person.