Cant figure this out!!?

ok this has been irritating me for a while now. kinda silly, but its a puzzler. sometimes after i wash and dry my clothes some of the t-shirts and pants come out with this odd stain that was not there before they were cleaned. looks like a little grease stain, or oil. no color but pretty visible. so now i look like a slob lol, and i dont know why. I have a 2 year old washer, i have used all temps of water with all kinds of variations of laundry soap. mostly tide cause others make me itchy, i use a good liquid fabric softener in the resevoir, in a downy ball...etc. i have even tried various temps and soaps and instead of liquid fab softener, using sheets. still happening and it is starting to ruin some of my best and favorite clothes. i'll give 10 points to anyone who can solve this puzzle for me. i have 4 loads of laundry ready to play around with your suggestions...... i'd appreciate it. id also like to know if i can fix the already stained clothes, if anyone can help me there!