I think what you may be refering to is a low red blood count (aka anemia). This means that the ratio of red blood cells to the total volume of blood is lower than it should be. The volume remains the same. In which case, the answer is yes- anemia affects menstrual flow- sometimes causing women to skip periods altogether. Iron deficiency is a common cause. Changing one's diet or supplimens are known to help.
If you did in fact mean less blood, I can only assume that would occur as a result of a bleeding disorder (such as hemophilia), but those are known to cause much heavier bleeding.
In any case, anemia is very easy to test for and treat (foods rich in iron, iron suppliments), but is not to be taken too lightly as it can cause other symptoms.
As for the pain, I can only tell you that it is not uncommon for a woman to have pain when she's starting her cycle, However, I cannot discern from your question the severity. Pain is normal- to an extent.
I would encourage you NOT to worry. It is unlikely that anything is seriously wrong with your girlfriend, but it would be irresponsible of me if I didn't tell you that it is always best to consult doctors for these things. They are the only ones capable of giving you a definitive answer/diagnosis. Best of luck!