You began by saying that you've been following Bernstein for 2 years and your control has been erratic for "the last couple of months." That means it was pretty good for the preceding 22 months, right? That's not what "failure" looks like, so for starters, stop beating yourself up. Doing what you've been doing, and the control it gave you, puts you up in the very highest percentiles of the diabetic population. You are not a failure; you're the opposite.
This is not a common cold, it's a serious chronic disease. There are going to be times when it behaves exactly like you want it to, and times when it resists and fights like a spoiled child. Ask the most well-controlled diabetic you know and they will tell you there are times when the numbers just won't do what they are "supposed" to.
You didn't indicate whether you are T1 or T2, or what your medications (if any) are. So if this persists long term without settling down, adding or changing meds might be something for you and your doc to look at.
As for causes . . . illness certainly is one, and you've had a few. So before getting too upset, get well and see whether things even out. Once you've been otherwise healthy for a while, see what happens then. And stress can really raise your BG, so worrying yourself sick (literally) about it just piles more fuel on the fire. Like I said, don't beat yourself up. It's useless.
And get that A1c tested! If it's better than you expect, it will improve your peace of mind, lower your stress level, and help you get back to where you want to be. If it isn't, then you'll know exactly where you stand and what needs to be done. Either way, you'll be able to get off dead center and keep moving forward. Ignoring it simply prolongs the anxiety, and helps nothing at all.