I doubt there is anything wrong.  The stems are elongating due to low light levels and are obviously bending toward the light.  You can rotate the plant toward the sun so the stems will grow more upright.  This is normal for the particular conditions.
When you put it outside, you will need to be careful because the extra sunlight could burn the leaves.  You may need to put the plant out on sunny days when the temperature is above freezing in the area where you place it.  Do not put it in direct sunlight to start with.  Place it on a north side of your house to minimize sun damage.  You will probably need to prune it back some since the stems will be a little week due to the excess growth.  This may happen naturally when you place it outside.  There will be some damage that will cause some stem die back.  New stems will grow that will be adapted to the outside sun.
You can gradually move it to direct sun, where most lilacs like to be.  Do a search on line for your particular variety to see if others can give you more information.