Hi Lucy :-)
Have you notified your pediatrician of your child's crying? You mention sweating a lot, perhaps your baby has an ear infection? I would phone the doctor right away. Also, you mention straining with his bowel movement, he may be constipated and needing more water even though you are breast feeding.. all things to bring to your pediatrician's attention right away.
There may be a benign reason as well for this crying such as early teething, but all in all I recommend you contact your pediatrician right away.
Once the baby is settled and the doctor has seen him, I offer that you visit my website www.amyfreundbodytalk.com and read about how you can learn to work WITH the energy of your son's body to help him be as balanced as happy as he can be.. you too!
There is a tab there that you can select which will teach you a FREE technique on how to balance your son's brain energy.. you can do this for yourself, too! The tab says Dr. Veltheim founder teaches CORTICES.. check out the two short videos and begin to practice this technique on yourself! It is wonderful for everyone so your son will enjoy it, too.
hugs,
A. www.amyfreundbodytalk.com