I've just started doing Romanian Deadlift for quite sometime and just yesterday I went heavy, I tried to have my chest out and arched my back, but it's hard when the weight is heavy, sometimes I felt I rounded my back when I lowered the BB... any tips for this?

and the next day I feel my lower back is rather stiff, a bit pain (never happened when I trained with medium weight.)

Is this normal fatigue or is it injury? How can you tell the difference?

stiff sounds like soreness to me. And if you think your back is rounding, then go lighter on the weight. If your back rounds you have reached techniqual faliure, you don't want this.

Thanks for the input. Strange... then I'm able squat heavier than deadlift... I read from somewhere that your DL>Squat>BenchP. Lol, o well I will go lighter on the Deadlift.

I can squat more than DL too.

Well, you said romanian deadlift. You usually do less for romanian deadlift than conventional deadlift.

Ahhh, that's why..

Use a belt.

I wouldn't use a belt unless I was going really heavy, like 90% 1rm. Personally, I never use a belt..
The belt keeps your natrual stabilizers (abs etc) from working to stabilize the spine like they should.