If there is too much bleeding you will often feel short of breath, dizzy, lightheaded, your pulse will be fast, and you will feel weak. Generally speaking, if you are completely soaking a heavy-days pad or a small chux pad in 30 minutes you are bleeding too much. Or passing a blood clot bigger than your fist.
Estimating blood loss is difficult, but if you keep all the towels or whatever you bleed on, you can weigh them to estimate loss 1 gram = 1 ml (minus the weight of the towel). Also you can use a measuring cup. 500 mls or more, or an amount of bleeding that makes you feel unwell, is too much.
But, at the time of birth, blood is often mixed with amniotic fluid, making it trickier to measure. Right after the birth there shouldn't be more than drops of blood coming out at a time-- if there is a steady trickle, it is too much. There will be a separation gush when the placenta is ready to come, but other than that gushes are a no-no. It is common to have little gushes when you get up or change position as the blood that was pooling in your vagina comes out, but if you are laying there or sitting there and there is a steady stream, it is too much.
If you are thinking there is too much blood but you are feeling well, put the baby to the breast or use nipple stimulation to bring your own oxytocin levels up. Also, feel for your fundus (top of the uterus) and make sure it is around your belly button and very hard. If it is soft, rub your belly (if the placenta is out) and make a contraction come. Also, remember to pee shortly after the birth, because a full bladder can impede your uterus from clamping down and keeping the bleeding to a minimum.
If you are feeling any of the symptoms of too much blood loss, call an ambulance right away.
Is that helpful?