My experience would say she's not being 'fussy' at least not in the sense that I would totally ignore the behaviour and think "too bad for you". In my experience, there's a reason when piggies stop eating something. It could be totally harmless - that she's just gone off lettuce for the moment. Guineas with their much better sense of smell and taste would notice if the lettuce isn't as tasty as usual even tho it might taste just like last month's lettuce for us humans. So could be that.
Or could be just a passing quirk

I don't have guineas any more, but when I did they sometimes went off a particular veg, herb, or wild plant for a while and then would go back to it. They like variety too I guess? One of my seniors, Suzie, suddenly started eating beet root and tomato around the time she developed tumors. I read later that altho beet root is not a medicine for cancer, it contains trace elements that are helpful, that maybe slow the process down, that kind of thing. She lived to 8 1/2yo despite her tumors and despite eating things which I now know from being on this forum that guineas shouldn't have more than once a week and ideally less often (like beet root and tomato), but I believe her nose told her what to eat. I had another elderly guinea pig who sat on the lawn eating buttercup leaves, which are poisonous. But I figured in her case that she knew what she was doing, she knew she needed some trace element from the plant. Normally I would have interfered and removed her, but she was surrounded by grass, clover and possibly bishop's weed aka ground elder but still decided to eat buttercups? OK fine, Emily, you know what you're doing. She made it to about 8yo as well, at least a year maybe two beyond the buttercup eating session.
To be clear: I'm not suggesting everybody start feeding their guineas things they shouldn't have but still - maybe sometimes some guineas know better than us, particularly senior guineas. I wouldn't let guineas near a really poisonous plant like fox-glove, but the problem with buttercups is when they are in your 'lawn', which resembles more a wild-flower meadow than lawn, you often can't find a spot w/o buttercups and clover, whereas it was never difficult to avoid any really poisonous plants like foxglove and lily-of-the-valley. Plus of all my guineas out on the lawn year after year, the only one who ever ate buttercups was Emily and just that one time.
I don't know about a specific health-related problem that not eating lettuce might suggest, but I am sure somebody with better knowledge will get back to you on that.