Your muscles store energy as a carbohydrate known as glycogen, allowing you to be able to carry out an intense workout as your glycogen stores are depleted. Refilling these glycogen-stores is one reason why post-workout carbohydrates are important.
Cardio also primarily uses this glycogen store as you burn calories on the treadmill. Therefore, doing cardio before your weight training sessions has been shown to have a negative effect on gains in strength.
Do you cardio after you are done with your weight training session for an optimal workout. Doing your cardio and strength training on separate days may be even better, but this has only been seen as trends in studies and more studies are needed to secure this fact.
Source 1: Cadore EL. Strength prior to endurance intra-session exercise sequence optimizes neuromuscular and cardiovascular gains in elderly men. Exp Gerontol. 2012 Feb;47(2):164-9. doi: 10.1016/j.exger.2011.11.013.
Source 2: Eklund D. Neuromuscular adaptations to different modes of combined strength and endurance training. Int J Sports Med. 2015 Feb;36(2):120-9.
Source 3: Cadore EL. Neuromuscular adaptations to concurrent training in the elderly: effects of intrasession exercise sequence. Age (Dordr). 2013 Jun;35(3):891-903.
Source 4: Wilson JM. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. J Strength Cond Res. 2012 Aug;26(8):2293-307.