Innocent flirting is innocent flirting! You two were dating before, and of course your comfort levels are different than just hanging out with another guy friend. You two have become accustomed to each other and the close contact that comes with it. If anything, it's really not TRULY flirting in my opinion, it's just you two being comfortable and buddy-buddy. It's hard to un-train yourself not to interact with someone that you used to always do before.
If you're worried about leaving your current boyfriend for your ex (not saying you are or aren't), then try to remember why you broke up with your ex. Has he changed? Is he mature? Think about everything you might be missing out on if your current boyfriend was out of the picture.
1 Year and 4 Months is a pretty healthy relationship..and after that point, "puppy love" starts to wear off. This is the crucial point in a relationship - can you handle the "real boyfriend"?
Prepare for some twists and turns and a more than bumpy road, because there will probably be more random little fights and things that never have bugged you will start to get at you. Just remember to take a deep breath and look at each situation.
Just to give you an example, I almost broke it off with my fiance a few months ago. I found out through my best friend, who he rants to and tries to talk to because she knows me best, that he's been spying on me through the internet and lied about a lot of things in our relationship. I was in hysterics, and was pretty much ready to break it off with him. Thanks to my friend, who told me to cool off, we (my fiance and I) figured out a solution to make the relationship work. Of course, he was deeply apologetic and he would do anything to keep me... so he took me to a theme park where we had our first date..it was cheesily romantic.
We're coming up on our 2-years-engaged mark..we're waiting until we're both done with our college degrees for marriage.
So, if anything, I would say try to spice up your relationship with your current boyfriend before worrying about your ex and what your friends comment about between you and your ex. If anything, try to hang out less with your ex and focus on bringing back something exciting into your current relationship.