what relaxer do you use? is it lye or no lye? and how long do you/have you been stretch(ing)? also, how do you wear your hair after you wash it?
i'm relaxed and when i stretch, i have the same issue, especially if am not roller setting my hair. some folks' hair is just prone to dread and i am one of them. when i was using a no-lye relaxer it was worse than it is now. now that i use a lye relaxer (mizani bb), it's much better but i still have to be careful not to stretch too long. other than that, what i do may sound slightly complicated but here it goes--
i use a good detangling leave-in and detangle my freshly washed hair in sections with a large tooth comb. then when my hair is slightly damp/almost dry, i moisturize it, seal it with one of my favorite creamy butters and/or JBCO and then detangle again first with a large tooth comb and then with a smaller (but not fine) tooth comb. then when my hair is completely dry i detangle again. for me, i only notice the tangled up knots of hair/dreads once my hair is fully dry so i have to do that final inspection to make sure i've fully detangled my hair. if when dry, i do encounter any dreads, which i always do, i detangle with my fingers first and then comb through the section with a regular (not large or fine) tooth comb from ends up to roots.
for my dread-prone hair, my no-no's are:
no lye relaxers (dry/tangle city)
not thoroughly moisturizing the roots and sealing them with a creamy butter/with JBCO
NOT fully detangling the roots (it will just get worse and worse)
only detangling with a wide-tooth comb (doesn't get all the tangles)
NOT detangling with my fingers
stretching too long