I have a lilac bush that is only 12" tall...a friend gave it to me and it was so small I put it in a pot with miracle grow soil...it was doing great all summer long till last week I notice some of the leaves had brown spots on them so I thought maybe I was watering to much...so I cut back on the water but the leaves r still getting brown and the ones from last week that had brown spots have now shriveled up an died...now we r in Missouri and the past 2 weeks its been very hot and very humid and it gets a lot of sun
Not sure what I'm doing wrong

Maybe it got too dry? Does the pot feel light? If so, you might try sticking it in a bucket of water for a couple of hours and let the whole root ball get soaked.

Do you have pictures? My lilacs are prone to mildew, and with the heat and being in pots.... they might get that although mildew doesn't present as brown spots.