Tell the truth, it does no harm.
Perhaps:
(a) I'm afraid that for the project we did last year I underestimated the amount of effort, and I ended up out of pocket, so I'm going to have to quote a more realistic price this time round.
or
(b) I was short of work last year, so I bid a very low price to be sure of getting the contract, partly because I knew that it would increase my visibility and reputation. I'm pleased to say that I've now got a wider range of clients, so I'm going to quote what I think is a fair rate for the job.
or
(c) I'm pleased to report that you're not the only client who has been appreciative of the work I completed for them, and I'm now unable to take on all the work that people are putting my way. This means that I'm now charging prices which I believe reflect the value of the work that I'm doing.