So I'm assuming you changed the given RAM in the launcher itself? If you didn't let me help you.
#1: Load up the modpack.
#2: Once your at the actual launcher (Before you click play and it loads up) You need to click installations
#3: Click On The Modpack.
#4: Scroll down and select more options.
#5: Scroll down again till you see JVM Arguements.
#6: Click on the second one, it will say -Xms256m- change that to -Xms5g- that will give you 5 gb of RAM, the higher number you put the more ram you get.