It's actually anything under a 3.5 that needs to be confirmed. I'd wait until 16 weeks have passed from your most recent exposure date, and retest. If it's still in the low positive range, then consider getting a Western Blot.
We can't answer why you have that result. Some experts think the number needs to be raised. If that's the case, then a 1.24 wouldn't actually be a low positive, it would be a definite negative. Try not to worry about it (yeah, I know), and get the retest done.