I cannot use artificial sweeteners at all. Those make my stomach hurt. It doesn't matter if I flare up or not. But coffee - maybe just listen to your guts? Usually coffee makes you go, so if your flaring up it can makes things worse. It is acid, so not so good for sick guts.
BTW, I drink usually very dark, strong coffee. But when my UC is off, I just can not even stand the smell of coffee