What Causes Irregular Brown Spotting While Trying To Conceive?

My husband and I are trying to get pregnant and my period is just over 1 week late, which is very rare for me, because it s almost always on time. I m currently on day 40 of my cycle and this morning I noticed some very light spotting, brown in colour, which has mostly disappeared as of now. I usually experience very painful cramps but haven t noticed any cramping at all with this spotting. I know i ll have to take a pregnancy test soon, but am very nervous to find out the results. Is there an explanation for this?