It can be difficult to differentiate between heavy
staining and a light flow. We follow the opinion that the sensation of external wetness is not considered a
hargashah. (How the
hargashot described in halachic literature really feel is hard to pinpoint.) In this case, given that it did not fully soak the liner and given that there is a significant possibility that it is from a
friable cervix, you are not considered
niddah.
Although we understand your reluctance to see your physician, bleeding such as you describe
during pregnancy should always at least be discussed with one's healthcare provider.
BeSha'ah tovah!