Is it potted?

It would be a process of elimination.

We need to know the care you give it, more details.

How wet is that mix, what it looks like, what kind of fertilizer, pH of your water and so forth.

Hi,

It is potted, the pH of the soil is 6.1.

I feed it with citrus feed every 2 weeks and change half of it's soil with new lemon potting mix in early spring/late winter every year.

This year it has been really wet outside, so it was pretty wet, however when I changed the soil the roots looked to be in good health.

What zone are you in? What temps did you have it while inside and when did you take it out (to what temps air/soil)? Since you said "every year", I'm thinking just give it some time for new growth to come on. I've never heard of "lemon potting mix", so can't comment on that mix, but obviously more yellowing leaves could easily be the water logged soil. Check the soil moisture level if necessary move under an overhang to keep precip.