I went through this SAME thing!
My son got on a solid sleep routine and started sleeping through the night between 8-10 weeks old and then when he was about 5 months old started waking several times in the middle of the night. We talked to our pediatrician about it and he told us not to feed the baby at night and that if we were comfortable with it, to let him cry it out. He said that babies need to learn how to put themselves back to sleep and if we kept running into his room every time he woke up, it would be a lot harder to deal with down the road (especailly when he moved into a bed).
The other issue was that he was still exclusively on breast milk. I had a checklist from the book Super Baby Food:
-Can your baby sit upright in a highchair?
-Does your baby seem interested when you or others eat?
-Does your baby pick up objects and bring them to his mouth?
-Has the baby who used to sleep long stretches suddenly stared waking up in the middle of the night?
If so... your baby is ready to add solids to his or her diet.
So we started feeding our son 1 or 2 small solid meals a day. His first foods were rice cereal, oatmeal, avocado and bananas (of course introduced one at a time).
We did listen to our doctor and we let him cry-it-out. It was really hard for me, but I kept looking at the clock and realized that he actually only cried for a few minutes and fell back asleep. I think that it was a combination of the extra calories, and learning to self-soothe, but after about a week, he stopped waking up (or at least, he stopped crying when he woke up and put himself back to sleep), and he has slept through the night ever since (he is now 15 months)
Good luck.