Definitely need more light. One "Gro-light" bulb in a lamp will not be enough for even a small lemon tree let alone as it grows. It will be twice it's size by next summer if you grow it well.
I'd go for a fluorescent fixture that holds 4ft. bulbs and use the T6 bright Daylight bulbs in it. Put it on a timer so you don't forget to turn it on and off, then in the afternoons during winter say 4pm or even earlier than that, have the timer turn it on for an extra 4 or 5 hours.
We're also assuming you have a full-sun south facing window for that plant to go through the winter? If not, it will need even longer.