Mine get hard when I get a little bit horny or while I masturbate. Most other times (including lovemaking) they are flaccid. I asked my doctor about this and he said it was due to the guilt we feel due to society placing judgement on these things and not to worry.
That phenomena happens whether you are pregnant or not. If you are looking for symptoms of pregnancy breast tenderness, nausea, bloating, skipped periods. The best way to tell is with a pregnancy test though :)