If you're 13 dpo on day 24 of a 28 day cycle then you must have ovulated quite early which might mean your period would come a little earlier. It's really difficult to tell, some people say IB happens around 7-9 dpo... I had it at 14 dpo but went on to miscarry so I don't really know if it was IB or just hormonal changes. I often get spotting before my period and it seems to be part of my normal cycle. I know it's hard, but you just have to wait and see I suppose because everyone's body is different and every cycle can be different. I try to keep a diary so I can see what's normal and what's not but generally I have a new symptom every month! Fingers crossed for you, if you're 13dpo tomorrow you should get an accurate result on a pregnancy test xx