Indoor light, no matter how bright, doesn't come anywhere close to what's outdoors.

I know nothing about lemons but doubtful it's possible to have a great specimen inside. They are full sun trees in a natural setting.

Oxalis is a bulb plant that can go dormant at times (of low light, low moisture, low temps, really high temps.) Do you know which Oxalis it is? Knowing where you are could help the discussion/advice to be as accurate and relevant as possible.

I have an update on my lemon tree problem. I had the soil tested & the pH was 4.6-apparantly I was overfeeding them (30-10-10) & the excess nitrogen was lowering the pH. Does this sound plausible? P & K levels were also very high.

you need to flush it with lots of water - sev times the volume of the pot. or put in the sink and run slowly water thru the pot for 10 min.
citrus forum has excellent info on growing citrus part indoors (winter) part outdoors. you need to repost there for feed/repot advice.
most citrus need to be outside during the warm months to do good. calamondin and meyer lemon can survive indoors in southern window. otherwise even they need to go outside. they can take part-sun - like 6 hours. but do better with more sun.