Kdd921 said:
I wouldn't normally be concerned but her blood glucose was 102 after 8 hours of the injection.
A +8 of 102 is fine

I would be more worrired if an early bg test, say +3, is 102 and you know the bgs are going to continue to drop until the nadir which is +6 or so.
Normal non-diabetic cat bg levels are roughly 60 to 150.
This is how Lantus typically works:
+0 - PreShot number.
+1 - Usually higher than PreShot number because of the last shot wearing off. May see a food spike in this number.
+2 - Often similar to the PreShot number.
+3 - Lower than the PreShot number, onset has started.
+4 - Lower.
+5 - Lower.
+6 - Nadir/Peak (the lowest number of cycle).
+7 - Surf (hang around the nadir number).
+8 - Slight rise.
+9 - Slight rise.
+10 - Rising.
+11 - Rising (may dip around +10 or +11).
+12 - PreShot number.
2 weeks ago when I checked it at this time it was 321. I am worried to even give her the shot tonight because I don't want it to get any lower. Is 102 too low? Should I test her again after she eats before I inject?
Blood glucose levels fluctuate all the time. You can test at the same time every single day but you'll always get different numbers. Food, stress, activity, etc can all affect bg numbers.
Always test before giving insulin no matter what. You don't want to blindly give insulin and risk a potential hypo a few hours later. Newbies should not give any insulin if the bg tests under 200.
Be sure to read the stickies on the Lantus board to understand how Lantus works for cats:
http://felinediabetes.com/FDMB/viewforum.php?f=9